I have been searching for an answer here and on google but can't seem to find a solution.

I have a Drupal 6.x multi-site install with multiple domains. It's a root install. One of my websites features a large forum running on phpBB in the directory /forum/. This works well, but now I want to use the Drupal forum on another website. Default for drupal forum directory is /forum also, so it won't work, it will keep showing the phpBB forum.

In other words:

http://www..xx/forum (here forum is a physical directory with phpBB)
http://www..xx/forum (Drupal forum, this won't work because it will point to the phpBB forum on www..xx/forum

I have tried an alias (message_boards -> forum). For the first page this works, but clicking on a link on the first page routes it back to /forum, thus ending up in my phpBB forum instead of Drupal forum.

Is there any way the default forum directory of Drupal can be changed? I really don't want to change the physical forum directory, so I must somehow do this either within htaccess or Drupal itself.

Thanks in advance for your help. Marcel.