Is it possible to mod an N64 controller to connect to a computer? I've read something about this online, and that the power ratings for the controller and USB are not the same. Is this possible? What do I need to do? Also, once connected with the proper amount of power, will I need a driver for it?
There are at least two ways of doing this, one easy way and one hard way.
The hard way (Do it yourself way):
Making a PIC board, which can support it. A reasonable tutorial to doing this can be found here.
You can buy a USB N64 controller ($10 on ebay - link removed: see below).
Don't buy them. I've bought many of these cheap N64 USB controllers, all from different sellers, and they're all crap. The analog always maps to the same controls as the D-pad, and isn't even true analog. I've had this even when the listing describes the controllers as having "true 3D analog" or some other crap. Buy at your own risk.
Instead I'd recommend modding your existing controllers, or just buying expensive USB ones from a reputable brand. If you do get stuck with a dysfunctional fake controller from ebay, file a dispute right away.
I'd say yeah, it is possible, but very improbable. You would most likely need to program yourself an N64 to USB/FireWire/Thunderbolt bridge yourself, and then either make it compatible with the HID for gamepads or write a driver for it yourself.
TL;DR Possible, but a great deal of work involving low level hardware drivers and programming a low level chipset. Sorry to be such a pessimist about it.