At the moment there isn't a good way to make folders launched by external programs open new tabs in an existing Opus window. A few people have asked for this so hopefully it'll be implemented in a future version.
If you want to try some of the imperfect existing solutions, have a look at these two threads:
resource.dopus.com/viewtopic.php ... ht=firefox
resource.dopus.com/viewtopic.php ... ht=firefox