I finally managed to add a 'Location:' row to the calendar-postsheet (basically, i duplicated all the 'event title' stuff in post.php&calendar.php&calendar.template.php and renamed it)
It works great, SMF is inserting and pulling the info from the database correctly and showing it in the Calendar..
But i just can't find the piece of code, that refills the form with the already existing information, when i click on the red star to edit the post...
Anyone got a hint please? I'm so blind... :(