I don't know if this is a defenitive solution because you might run into trouble when interfacing the analog sticks.
This will require an awful amount of wires and you'll end up having 1 N64 controller for the Xbox that will work only on the Xbox. IMO the way that would require the least amount of intellectual effort would be to simply sacrifice a Xbox controller for it's mainboard and map each button input of the N64 controller directly to the corresponding button input on the Xbox controller. I don't think any N64 emulators on the Xbox could make use of a Computer-N64 controller adapter directly (let it be USB or any other type).
