How do you create polymorphic ringtones for 720/730

I just bought a 730 series from motorola. Been playing around with
the composer, which is nice, but I want to know how I can create my
own polymorphic ring tones. There are a number of sites that sell
them for 2 bucks each, but is there software you can buy or some
conversion program that takes wav or midi files and outputs the right
format? Also, even if I do create it, how do I get a polymorphic ring
tone to my phone and save it correctly?

thanks, t