Welecome to FT... Unfortunately, you cannot book with or pay UA for the flight. You have to book with AC. It should work like cash. If your fare is 500.00 you would have 300.00 out of pocket + the voucher.
Consider a weekend trip from SFO-YYV, YYZ, and have the flights credited to MP. The most inexpensive option would be SFO-YYC about US$260.00 so a trip to Calgary should only cost 60.00 out of pocket.