Step 1: Plug in a keyboard from another computer
Step 2: Change your controls
Step 3: Unplug keyboard and be happy.
You do so have a keyboard.... you typed this message didn't you?

Keyboards cost as little as a couple of bucks at this point. Why do you think every single mame cab ever built has a keyboard stuffed in the coin door?
Also if you have joysticks then you obviously have other input methods. If you didn't make an admin button I don't know how you expect to get into the menu without one.
You can easily remap the admin button from tab to something else, but you first have to press tab to bring up the menu.