How to add namespaces in web.config file?

You need to put them in the correct <system.web> section. e.g.:

<configuration>
  <system.web>
    <pages>
      <namespaces>
        <add namespace="System.Data" />
        <add namespace="System.Text"/>
      </namespaces>
    </pages>  
  </system.web>
</configuration>

and put them in the correct web.config

i.e. the second web.config file is the Views folder and is specific to views. These settings do not go in the root web.config.

The purpose of these settings is to make the libraries available to the ASPX pages (e.g. for Intellisense) and it is not used for the code-behind. You still need to have using statements in your actual code as that is just plain c# programming.

The purpose of the namespace section is to get around having to do the import in the .aspx page. Code behind in C# still requires you to have the using statements at the top of your .cs file.

There is no way to get around this.