When you book UA flights on Miles and Smiles website, M&M number sticks to the booking, and there is no way to change it to MileagePlus number on TK or UA website. Is there anyway around this?
This is similar to the issue that exist with booking AA flight on BA website, in which BA number sticks to the ticket, and there is no way to change it. But there are ways to change AA to BA number by retrieving the booking on third party airlines. There also ways to avoid the issue at the first place with AA bookings on BA. But I am not ware of any method to eventually add a MileagePlus number to a UA metal award ticket booked by TK miles.
Call or tweet UA. No other options AFAIK.
In the future, the other trick is to book your reward flights not as "member" but as "book for someone else" yet this third party happens to share your name and other pertinent details but has the frequent flyer number you desire. (might not be relevant to TK however)