There is no "mp3 to DS" converter. Heck, there is no "mp3 to GBA converter" either. GSM player converts your WAVE audio files to GSM audio files, then attaches files together, player + song1 + song2+ song3 etc. MusicPlayer Advance does the same thing. In both cases, the audio files are converted to a different format and played back by a GBA program that can decode that format. You're not converting to any native GBA audio format or anything.
I suppose if you want to get something similar, you could get Lick's old Helix MP3 player demo, in which you embed a low-bitrate MP3 file into a *.nds file which then plays it back, but again, you're not converting to DS format or anything.