There are no direct methods to transfer between any of the *A programs. With UA miles, you are locked into what UA offers and inventory its partners release to UA. Availability on partner flights at other programs does not necessarily equate to same availability at UA.

Your best bet would be transfers from credit card currencies into airline and hotel programs. Most of these programs transfer at 1:1.

You might want check AC and review its inventory of SQ (hint hint). AC and SQ play much nicer than the tumultuous relationship between UA and SQ.