Retrofit2: Modifying request body in OkHttp Interceptor

I am using Retrofit 2 (2.0.0-beta3) with OkHttp client in Android application and so far everything going great. But currently I am facing issue with OkHttp Interceptor. The server I am communicating with is taking access token in body of request, so when I intercept the request to add auth token or in authenticate method of Authenticator when I need to add updated auth token, I need to modify body of request for this purpose. But it looks like I can only add data in headers but not in the body of ongoing request. The code I have written so far is as follows:

client.interceptors().add(new Interceptor() {
            @Override
            public Response intercept(Chain chain) throws IOException {
                Request request = chain.request();
                if (UserPreferences.ACCESS_TOKEN != null) {
                    // need to add this access token in request body as encoded form field instead of header
                    request = request.newBuilder()
                            .header("access_token", UserPreferences.ACCESS_TOKEN))
                            .method(request.method(), request.body())
                            .build();
                }
                Response response = chain.proceed(request);
                return response;
            }
        });

Can anyone point me to the right direction as how to modify request body to add my access token (first time or updated after token refresh)? Any pointer to right direction would be appreciated.

I using this to add post parameter to the existing ones.

 OkHttpClient client = new OkHttpClient.Builder()
                    .protocols(protocols)
                    .addInterceptor(new Interceptor() {
                        @Override
                        public Response intercept(Chain chain) throws IOException {
                            Request request = chain.request();
                            Request.Builder requestBuilder = request.newBuilder();
RequestBody formBody = new FormEncodingBuilder()
            .add("email", "[email protected]")
            .add("tel", "90301171XX")
            .build();
                            String postBodyString = Utils.bodyToString(request.body());
                            postBodyString += ((postBodyString.length() > 0) ? "&" : "") +  Utils.bodyToString(formBody);
                            request = requestBuilder
                                    .post(RequestBody.create(MediaType.parse("application/x-www-form-urlencoded;charset=UTF-8"), postBodyString))
                                    .build();
                            return chain.proceed(request);
                        }
                    })
                    .build();

public static String bodyToString(final RequestBody request){
        try {
            final RequestBody copy = request;
            final Buffer buffer = new Buffer();
            if(copy != null)
                copy.writeTo(buffer);
            else
                return "";
            return buffer.readUtf8();
        }
        catch (final IOException e) {
            return "did not work";
        }
    }

OkHttp3:

RequestBody formBody = new FormBody.Builder()
                .add("email", "[email protected]")
                .add("tel", "90301171XX")
                .build();

I'll share my Kotlin implementation of @Fabian's answer using Dagger. I wanted origin=app added to the request url for GET requests, and added to the body for form-encoded POST requests

@Provides
@Singleton
fun providesRequestInterceptor() =
        Interceptor {
            val request = it.request()

            it.proceed(when (request.method()) {
                "GET" -> {
                    val url = request.url()
                    request.newBuilder()
                            .url(url.newBuilder()
                                    .addQueryParameter("origin", "app")
                                    .build())
                            .build()
                }
                "POST" -> {
                    val body = request.body()
                    request.newBuilder()
                            .post(RequestBody.create(body?.contentType(),
                                    body.bodyToString() + "&origin=app"))
                            .build()
                }
                else -> request
            })
        }

fun RequestBody?.bodyToString(): String {
    if (this == null) return ""
    val buffer = okio.Buffer()
    writeTo(buffer)
    return buffer.readUtf8()
}

Since this cannot be written in the comments of the previous answer by @Fabian, I am posting this one as separate answer. This answer deals with both "application/json" as well as form data.

import android.content.Context;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;

import okhttp3.FormBody;
import okhttp3.Interceptor;
import okhttp3.MediaType;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;
import okio.Buffer;

/**
 * Created by debanjan on 16/4/17.
 */

public class TokenInterceptor implements Interceptor {
    private Context context; //This is here because I needed it for some other cause 

    //private static final String TOKEN_IDENTIFIER = "token_id";
    public TokenInterceptor(Context context) {
        this.context = context;
    }

    @Override
    public Response intercept(Chain chain) throws IOException {
        Request request = chain.request();
        RequestBody requestBody = request.body();
        String token = "toku";//whatever or however you get it.
        String subtype = requestBody.contentType().subtype();
        if(subtype.contains("json")){
            requestBody = processApplicationJsonRequestBody(requestBody, token);
        }
        else if(subtype.contains("form")){
            requestBody = processFormDataRequestBody(requestBody, token);
        }
        if(requestBody != null) {
            Request.Builder requestBuilder = request.newBuilder();
            request = requestBuilder
                    .post(requestBody)
                    .build();
        }

        return chain.proceed(request);
    }
    private String bodyToString(final RequestBody request){
        try {
            final RequestBody copy = request;
            final Buffer buffer = new Buffer();
            if(copy != null)
                copy.writeTo(buffer);
            else
                return "";
            return buffer.readUtf8();
        }
        catch (final IOException e) {
            return "did not work";
        }
    }
    private RequestBody processApplicationJsonRequestBody(RequestBody requestBody,String token){
        String customReq = bodyToString(requestBody);
        try {
            JSONObject obj = new JSONObject(customReq);
            obj.put("token", token);
            return RequestBody.create(requestBody.contentType(), obj.toString());
        } catch (JSONException e) {
            e.printStackTrace();
        }
        return null;
    }
    private RequestBody processFormDataRequestBody(RequestBody requestBody, String token){
        RequestBody formBody = new FormBody.Builder()
                .add("token", token)
                .build();
        String postBodyString = bodyToString(requestBody);
        postBodyString += ((postBodyString.length() > 0) ? "&" : "") +  bodyToString(formBody);
        return RequestBody.create(requestBody.contentType(), postBodyString);
    }

}