Q: transfer account balance to another account

You can't transfer it yourself, but you can try contacting iTunes support and see if they can help : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

I think you can just transfer it as a gift. Maybe not what you initially intended, but I think its the same result. Go to the page below for an explaination. You can either gift the dollar amount, or just buy something under your old account and gift it to your other account.

http://support.apple.com/kb/HT2736?viewlocale=en_US

Ignore the previous post, itunes won't let you purchase gifts with account balances.