It's probably not that easy, first you'd have to know what type of signal, rate and voltage levels the pad uses.
Then you'd need some electric components to adjust the usb voltage levels to fit the n64 pad. You'd need to design an integrated circuit that recieves, interprets, translates, stores and then sends the signal to the usb port.
Finally you'd also need a usb driver for those translated signals, unless the circuit pretends to be some other standard joypad that the operating system identifies.
I probably missed something, but it would be cheaper and easier to cough up the $10 or whatever one costs instead.
There are some things better left to professionals. You can an adapter that works for both N64 and PSX controllers at Lik-Sang or Play-Asia I'm pretty sure. (don't quote me on it though, I'm just assuming they still have them.)
Creating USB devices is much more complex than creating a parallel port device. And if you only want to save money, you'd actually end up spending more making your own unless you happened to have the components that you needed already.
Yesterday I placed an order on this site.. but today checked the forums and there's a lot of people complaining about putting orders on the site and not receiving anything (being robed), not getting answer to their e-mails.
I placed an e-mail yesterday asking to cancel an order cause by mistake placed two orders of the same product, it's now and haven't received any replies, and the 2 transaction are authorized on my credit card,
So, if you read this please check the site forums first and see it yourselft, I personally feel fooled by now, cause they already charged my credit card, I'm thinking in going tomorrow and block it, change the number, the as**oles in that site are thieft.