From Birnam Designs Wiki

The most reliable method seems to be to save the mp3 to flash memory, and then use the phone to send it as a media attachment to itself. The original cannot be used as a ringtone, but it can once it is received and (re)saved from a multimedia message.

Alternatively, here is a more elaborate method that yields mixed results:

1. Make sure your Transflash card is in your phone.
2. On your phone go to "Get Tunes & Tones"
3. Select "Record New"
4. Select "Options" and switch the storage device to "mini-SD Card"
5. Go back to "Record New" and record a simple audio. It does not matter how long, I did it for 2 seconds.
6. Go back to "Get Tunes & Tones" and scroll down to the bottom and play (to make sure it recorded something) the new audio recording.
7. Shutdown the phone and put the Transflash into a PC.
8. Copy a mp3 file into the root directory of the Transflash card.
9. select one of the recorded voice file, right click, rename, copy the file name including the extension (.qcp)
10. Delete this recorded file you just copied
11. now go to one of the mp3 files, right click, rename, and paste that recorded file you just copied (all you're doing is replacing the voice file you've recorded as an mp3 audio file. Your cell phone will still think it's a recorded voice files but in reality it's an mp3 audio file)
12. Repeat step 3-11 for every ringtone you want to add. One voice recording = One ring tone.
13. Once the Transflash is back into the phone assign the file as a ringtone to any contact.