It doesn't matter if you buy from CO or not for credit. It also doesn't matter if you have a CO code on the flight for SWU/Cash+Miles upgrades.

That said, if you still want to do it you can. Book the CO metal segments on co.com and choose the pay with cash option. Then call the co.com web support desk and have them add the OAL segments in to the itinerary and price it. You can then complete the transaction online.