I haven't encountered your situation-- having a oneway booked, but wanting it to be a roundtrip to unlock the open jaws and stopover that are necessary for the free oneway.
I don't have an answer about whether United will let you book another oneway, then combine them into a roundtrip, then let you add a free oneway. I would think they would make you cancel your oneway at a cost of $150. Probably not worth it for a oneway LAX-SFO.
For anyone who missed my post in this forum about free oneways. It is apparently now post #7 in this thread:
http://www.flyertalk.com/forum/unite...ard-miles.html