You just have to tell DirectoryServices to reload it’s configuration files:
killall -HUP DirecotyService
This must be run with root permissions (so: with sudo), and you don’t want to forget the ‘-HUP’ as without that DirectoryService will sometimes erase your changes.
But since this is a InstaDMG conversation, you don’t have to do this, since DirectoryServices on the target volume is not running, and will pick up things when you boot it for the first time.