I managed to put some mp3 songs on my desire.
I plugged into the USB of my laptop, set Desire to be a Disk Drive it mapped as drive F:
i opened the folder MP3 from drive F: and say a few folders (preloadedmusic)
created the Lady gaga Folder and stuck all the MP3's into it.
closed app
started Music app and listed songs, hey presto Lady Gaga listed.
Simples...;-)