Trying to auto-register a Nagios NSCA client
I don't know if OP would still care about this question six years later, but in case anyone else stumbles upon this there's a tool for converting passive checks into new hosts. It works by watching the Nagios log for errors from passive checks associated with an unknown host and creating the host.
https://exchange.nagios.org/directory/Addons/Passive-Checks/Radar--2D-add-hosts-and-services-automatically/details