HTML is disabled by default in miniBB, and we don't recommend to enable it, for security purposes. Besides simply adding the button, you need to care a lot of things - that's right, in bb_codes.php, too.
By default, miniBB has a hidden option, under setup_options.php you may specify:
it will allow direct HTML posts for admins and moderators.
If you mean the special BB code which would allow certain pieces of HTML in the posted messages, I'm afraid it's even more difficult :-) We don't have such code right now.