I understand completely and what you want to do is totally reasonable. The only caveat particular to my question is whether that Express is one that can even be used these days:
I purchased what I thought was another AirPort Express to add to my network....
The original AirPort Express models bore designations A1084 or A1088 and that's what you need to rule out first. They won't work, and their value on the used market is essentially nil.
Unfortunately they are physically identical and indistinguishable to the later model A1264, which — should — work.
The later Express models have a "flat, square" appearance. They should work also.
Resolve that uncertainty first, and then we can address adding the Canon printer to your network.