I'm a few miles short of what I need to get vacation tickets for the whole family (I need 300K, I have 275).

Between the wife and kids, I have enough miles to cover the difference, but I'm unable to combine these with the ones in my account according to the rep at Mileage Plus.

Is this truly not possible or am I just not trying hard enough?

BTW, it must be the economy - in the past, I'd buy the tickets and try to figure out how to max the miles posted to my account instead :-)