I recently ran in to an issue where I could not use an external IPAM provider and needed to use the native one in vRA. The only problem was that I could not do IP reservations in the ranges declared in the network profiles. With the blessing of vmware support I have written a tool to reserve IP addresses in the native IPAM. It is still a beta and I am not done with it yet, but if it helps anyone it is here for you:
GitHub - steskalja/vRAIPReservation: This tool allows for IP reservations in vRealize Automations na...