I was just wondering if there's a way to merge multiple user accounts into one.
Not sure if you mean on this site (if you have another account) or if you mean in myBB generally. So I'll presume you mean the latter. Yes, in ACP you can do
http://i.imgur.com/88dFU.png
Just do that multiple times. It's not difficult, nor time consuming ... :/
Merging multiple users at once needs a plugin.