Sorry if this is in the wrong place.
I know this isn't anything new and most people will probably know this anyway, but I've told this to a few friends and other people they never knew so I'll just say it here.
If like me you've had friends who've quit the game and given you their account, (you can unconnect accounts to battlenet by contacting Blizzard via email (you'd have to ask your friend to do this) and they'll do it for you), then simply connect it to your own battlenet account, as long as 2 accounts have been connected to the same battlenet for over 1 month then you can transfer characters from 1 account to the other regardless of the details of either account.
I've transfered about 4 characters from 3 different accounts to my own account.