This one is a little tricky, but it can be done. It can be accomplished in two ways, one that I don't recommend and one that is the hardest but cleanest way to do so:
1) Use Zedge - full of ads, download it from the store, chose some nice ring tones.. Then uninstal Zedge.
2) Use a ring tone maker or a sound editor on your computer to convert sounds to the ogg format. Then create a folder in your phone and upload the ogg sounds on it. The system will recognize them immediately as system sounds.
