... it by username not number as the Joomla and phpbb3 usernumers are different. This works fine until you get a username with an underscore. In the case above, going to site.name/......../john_smith will just give the user doesn't exist error. You need to manually set up redirects for each member ...

... in joomla and in phpbb? Yes, possible, after transferring users from Joomla to phpBB and vice-versa, if you don't have same users (users with same case-insensitive username or email). I.e. bridge can't copy Joomla user 'Admin' to phpBB if phpBB already has user 'admin'. Is it possible to copy to ...

if that is the case is it possible to just move the contents of the Joomla directory to the root directory and things won't break?

i don't want to move the files and break the site as I've already installed a lot of components