That doesn't matter
Just install your forum, and if a user wants to have his domain setup, just add a Co-Domain, and enter as forwarding target e.g. "http://forumdomain.com/user1"
This path won't be created and there won't be any additional user accounts created.
Now I'm unsure what your question is...
a) Do you want the domains to point to these (already existing) subfolders that "load" the different tables?
b) Do you want to create these folders automatically?
If a) -> no problem. My suggestions 1) and 2) both should work.
If b): You need a solution independant from ISPConfig. You can of course still use ISPConfig to redirect domains to subfolders, but for creating these (possibly virtual) folders you need something else.
You could e.g. create ONE php file that accepts a GET-parameter containing the table prefix (ATTENTION: Be aware of SQL injection attacks!) and redirect virtual folders with the help of a .htaccess file to this php file, automatically filling the parameter.