You could do that only you would have to buy 2 months for the RAF account but your main account would get one month free so it would work out to the same price.
Battlechest for $25
1x Gamecard for $30 (Put the time on the account as soon as you activate the CD key)
I had him use a gamecard because i saw where a few people had issues with waiting for payments to clear.
You may be forced to wait up to 7 days until your RAF trial is upgraded to a full account after purchase of the game, but it can be as fast as a few hours. Depends on server traffic.