You can add the switch - it has been done and I don't think there was any issue with the digital signal. Try to keep the leads as short as you can. I think it's a DPDT switch than can open one circuit while closing the other.
In fact, I think you could even leave the resistor in place and only put a switch on the jumper. As long as you don't try to run the s/pdif out at the same time you have a computer hooked up (keep the switch open), it should work fine.
Not sure what would happen if you had the switch closed - you can test it by manually making the jump on your open UA-5 before doing anything else.