It is true, that you are able to use MP3 as ringingtones in N-Gage. I wonder how you were able to send anything to N-Gage via infrared, because N-Gage doesn't support infrared.... Are you sure that a) you have N-Gage, b) it was infrared you were using?
Anyway.... no matter what the transfer technology was, you should be able to send files to terminal by using USB-cable and Nokia tools.