Maybe you missed it above, so I'll say it again since it's worth repeating: you cannot transfer the miles you've already earned in the Asiana program anywhere. We are talking about using UA for future mileage accrual only.

One other point to be aware of: on UA the bonus miles you receive as an elite are only RDMs (redeemable miles, good for award tickets, upgrades, etc.) There are no bonus EQMs (elite qualifying miles, which is how you determine whether you get status in the following year). Earlier you made a comment that leads me to believe you think the bonus miles are EQMs too,..they are not. Everyone has the same requirement to earn EQMs for status and there are no bonus EQMs, generally speaking, for current elites.

I would second the advice given - stick with the program of whichever airline you think you'll fly more.