Just an FYI, all you have to do is click Merge WoW account to get more than one in there. I asked them the same question. They did say it is capped at 8 WoW accounts per battle.net account however. I am unsure if there is a way to merge battle.net accounts however. I asked that as well and have not gotten a response as of yet.

Might also be worth while to note that when you merge multiple wow accounts into a single battle net account you have to explicitly state that you are the sole owner and player of both accounts.