It's obviously a hybrid, allready heavily modded version of the phpbb boards. Have a look at
www.phpbb.com and
www.phpbbhacks.com and see if anything on there might be a use.
May be of use posting on the support forum of the modded board first to see what they have to say. (looks like they are at
http://www.phpbb2.de/ )
Before any attempt at adding anything, I'd ensure you have full backups of the database and web files incase it all goes pearshaped. Indeed might be easier to create a test forum, totally separate from you live one to try things out first.