"Is it possible to mod/hack your mac mini so it has usb 3.0 ports instead?"
Short answer:
No, not really.
Longer answer:
It -might- be possible to kludge together some kind of solution, but by the time you had it working, you'd do better to spend the extra $$$ (probably not much) just to buy a new Mini, and then sell the old one for whatever you could get for it!
USB3 is the -singularly most important- reason to wait for the upcoming 2012 version of the Mini (and other Macs, as well).