this is unfortunately not possible today. IPAM requires direct visibility to those networks/ DHCP servers (like VPN tunnels). Additional poller is also not the best option as we understood from the users they want to use it more as "remote poller" not because of they running out if max. IP licenses per single machine.
We have remote poller as feature request and working on Orion platform solution for the future, that should solve this problem.
What's the status on adding IPAM polling to the Additional Polling Engines? We need to be able to initiate IPAM polls from an APE that sits behind a firewall, so polling from the Primary Orion won't work. This feature seems to have been on the road map for a while. I'm curious if you can provide an update on this feature request?