Playing with the converters, I did convert a .mp3 to .mod - But at a rather severe cost in quality.

The way I did it was to convert the MP3 into a wave file (Winamp is good at this) while adjusting the converter's quality to lower the file size (wav's are huge files. don't be shocked if you see a 30mb wav come out!).

Why go through this step? Wavs can be loaded into modplug. Save it as a mod, make sure the size is AT LEAST below 2.8mb (Otherwise jj2 will get amnesia errors), then load the song into jj2.

Listen to the whole song to ensure it loops right - converted wavs have a constant habit of looping mid-song. The trick to fix this is to add more playing space and slow the script speed. This will require some constant tinkering. The result is a (Grainy) song that is still rather large in size, that won't loop right in winamp, but loops okay in JJ2.

Time and quality wise though, Unless you are willing to recompose the song into modplug (You need to know how to play a keyboard!), and until a MP3 add-on comes for JJ2, it's easier to search the Mod Archive and find a good substitue song.