Im not sure if this was posted before, tried searching and found nothing. Could be common sense but first time i ever noticed.
My mate basicly wanted to start playing the game so i gathered the expansions etc and so on to get this baby going. Thing is he had a trial account which means it has 10 days before it expires, so i merged the expansions and the original game with it and viola. It had 30 days from Vanilla wow applied to the 10 days = 40 days without trial restrictions
So in steps
Make trial account right before you merge the original game/expansions with it
Merge them
???
Profit