Is it possible to get the usernames and their passwords out of the ISPconfig database (?), so that they can change in ISPconfig their passwords and can login with their ISPconfig username and password through the .htaccess checkback infront of the intern forum?
That depends on the login information that you need.
The usernames and passwords are stored in the database table sys_user and you can retrieve it from there.
The passwords for the email users are not stored in the ISPConfig database. If you gave to validate against them, you will have to retrive them from /etc/passwd and /etc/shadow.