I didn’t know that there was a list of Disney ringtones posted on the forum. I wish I knew this before!
If you have in mind the same one that I found [here] (Disney Ringtones? - #6 by KariBelle), it looks like the files are already formatted as ringtones, which should simplify the process a lot. It would still require you to sync with your computer, though, I believe. You should be able to drop the ringtones you want into iTunes on your PC or Mac, on iTunes make sure to check the ones you want, and then sync your phone.
I’m not at home right now, so I can’t test this out. But I am planning to do so when I have a chance.