You can do this with original mp3's , Ill tell you how:
1. Connect your PSP to your computer.
2. Go into "PSP"
3. Now go into "SAVEDATA"
4. Make a new folder called "ULUS10041CUSTOMTRACKS" if your in the US.
"ULES00151CUSTOMTRACKS" if your in Europe/Australia or whatever.
5.Now you must convert you MP3 into ".gta"
6. So Download "Exact Audio copy" I cant provide link (try google "PSP Exact audio copy)
After it install (choose manually restart computer)
7. Open the program.
8. Drag and Drop the MP3 you wish to use.
9. Once that is done get you file that the program converted it to.
10. Change the end of the file to ".gta"
11. Now put this in the folder you made in Step 4.
12. Start Up GTA LCS and go to "audio" and change the options so it supports Custom Soundtracks