Looking at the performas service manual, using the CRT to connect to both the Mac and N64 is going to be damned near impossible.Having said that, I'm assuming the connections between the logic, analogue video and video boards are all machine specific. The manual doesn't give a good enough view of the connectors.
Mactracker says the 5215 and 5220 have built in RGB CRTs and/or optional DB-15 display connectors. Whether these DB-15s are external or not I'm not sure.

If this is correct, you might be able to find a way to connect the N64. But I suspect it will involve a chain of antiquated adaptors to convert the signals at the very least.

Have you considered running the N64 through the performas TV Tuner? Might be easier, though obviously not ideal....