user title moderators only in the forum they moderate
 

i want a hack which shows the user title moderator just on the forums they are moderators
and in the other forums, the normal user title evolving with the number of posts is shown

someone knows how to do it ?

That's no good idea, because usertitles are saved in user-table.
if you want to change usertitles forumbased, you have to add extra queries to getpostbit function, and that will increase your query amount a lot.
perhaps there is an easier method out there with just one extra query per page, when i found out one i'll tell you, but i don't think ;)