I wrote a program today for myself that helped me make my custom radio without having to copy-paste literally every mp3 I wanted in my radio. Essentially what the program does is create symbolic links from your mp3 file to the /mp3 folder for your game (or wherever you specfy, really).
It only works in Windows from Vista onwards, since XP and previous don't have the mklink command. I figured I'd release it considering I got some use out of it. If there's breaking bugs I'll give a go at fixing them. Cheers.
The source code is available at: http://github.com/megadrive/gta3vcmusic
Latest downloads found here: https://github.com/megadrive/gta3vcmusic/releases/latest
New update! v0.2.0, adding playlists! Changelog and downloads here: https://github.com/megadrive/gta3vcmusic/releases/latest
Another update, it now saves your last used directory used and you can now randomise your playlists.
edit: Randomising playlists may not do anything as yet, will update.