Having tried to do this some years ago I remember it not being very easy.
The way I did it was to create totally new table to user/pass and then replace these columns in the respective software with foreign keys to your new table - this required a lot of custom tweaking of core files in each application - mainly making sure all SQL requests to this data have the extra join needed for your new table. If I find the time I will maybe try and provide a step by step of the changes needed.
There are some pretty big drawbacks to this approach though. The main one being from now on your gonna have to hand update any patches
If you have no content or users yet look at http://bbpress.org/documentation/integration-with-wordpress/ which will make things a lot simpler for you.
I can't quite remember but I believe that I big problem I had was that MediaWiki requires usernames formatted a certain that conflicted with phpBB.
Of course, a totally different approach would be to mod each piece of software to use OpenID _ I believe plugins/extensions are readily available for all the applications you mentioned.