That does sound odd, but the obvious solution is just to book the itinerary on the AC website. If you are trying to use your AC status to upgrade and then credit to UA, you will probably have to enter your AC FF# and then try to get the gate agent to change it to UA after you check in -- which they may or may not be willing to do.

If your main interest is upgrades, I would look at AA. I have no direct experience as I'm not elite on UA, but what I've heard leads me to believe that AA EXPs have better upgrade chances than UA 1Ks. But that's only relevant if AA's route network will serve your needs. My guess is you'd be connecting a lot more than you were on AC.