It is not possible to modify the Apple Radio station list that is available within iTunes or via Home Sharing.  However, there are two ways of importing custom Radio stations into iTunes:

 

Import an M3U or PLS playlist file

Many radio web sites publish their radio stream as an M3U or PLS playlist file.  These files can be imported into iTunes when running interactively - either by logging in via Remote Desktop Connection or by running iTunes interactively from within the iHomeServer console.  You can import the file directly into a custom playlist called, say, 'Custom Radio Stations'.

 

Note that although iHomeServer supports importing M3U files, it currently does not support importing M3U files that contain URL streams, such as radio station M3U files.

 

Import a direct HTTP radio URL stream

The M3U file mentioned above is usually a wrapper for the direct URL stream of the radio station.  iHomeServer allows you to add an internet stream directly to an iTunes playlist using the Add URL Track to Playlist window.  Therefore, if you know the URL of the radio station audio stream (or you open the M3U/PLS file in a text editor and extract it manually) then you can add the radio station directly from within the iHomeServer interface.