Can I use a GitHub project directly in Maven?

Try jitpack, you just need to add the dependency, jitpack will build others for you.

From home page:

jitpack
Easy to use package repository for Gradle and Maven projects
JitPack builds GitHub projects on demand and provides ready-to-use packages

HOW

1. Add repository first

<repository>
    <id>jitpack.io</id>
    <url>https://jitpack.io</url>
</repository>

2. Add dependency

<dependency>
    <groupId>com.github.User</groupId>
    <artifactId>Repo name</artifactId>
    <version>Release tag</version>
</dependency>

TIPS:

You can see its build log too https://jitpack.io/com/github/NanoHttpd/nanohttpd/Release-2.1.0/build.log

Not in the way I think you mean, AFAIK.

You can use github as a Maven repository--this is not the same thing as directly referencing a project, and that it has a pom file means only that it's a Maven project.

If the project is not available in the central, or other, repository, your best bet may be to clone it, build it, and install it locally. You should confirm that it's truly not available elsewhere.