Can I transition from aluminum wiring to copper wiring at a CO/ALR receptacle?
The manufacturer of the receptacle should make this detail clear in their documentation. It might even be printed on the case of the receptacle, though perhaps in very abbreviated / "hieroglyphic" form.

That said...

My understanding is that aluminum wiring must be used ONLY with approved mechanical connectors specifically designed for aluminum, and of course must be installed correctly. So I do not see, offhand, why a separate copper connection on the same device would be a problem.

[Note - I am not an electrician. Aluminum wire done incorrectly is a huge fire hazard, which the OP obviously knows since s/he is obviously intending to do this right.]