If you forum is unstalled in a sub-directory (eg /forum/) within your web site, then you just will need to install joomla at root, install cb, and then install the plub-in.
Once you put the 'correct' path to your forum into the phpBBConnector variable, it'll all work fine and you'll be able to import all the users into Joomla/CB.
Do you have any phpBB hacks installed. See, mine has LOADS of hacks, many hard coded, so I am having to install a fresh copy of phpBB, and then import the data, but if yours is bog standard, you shouldn't even need to do that.
If your phpBB is in the root folder of your wespace, then you'll need to move it into a folder (/forum/) and install joomla at root instead, but its not a great deal of hassle.
But back-up everything, including the database before you start anyway!
Regards
Paul