If you eventual point in all this is that you want to use a USB gamepad to play Speedball 2, then this is nigh-on impossible.
Yes, you can attach a USB port to a 68000, the rest of the system is just code. However due to the nature of the software part of USB, you would need to run what is probably quite a heavy duty bit of software alongside the game that already probably uses >90% of the 68000's available power. Alongside that you would need to map the USB gamepad input into standard Amiga joystick inputs. And then you'd realise that Amiga games boot from ADFs on the MiniMig, and that there's no way to stick the USB stuff into that.
The only way it could be done would be to have a second CPU that runs the USB stack transparently, and also does the mappings to classic Amiga hardware transparently. Here, of course, you start seeing the level of work that would be required.