Hi Oskar,
because IPAM licensing is based on "used" IP Addresses you may create - pre-populate your subnets with all "Available IPs" in it and then let IPAM to keep scanning the subnets. if it finds newly used IP address you may get an alert and for licensing is just "-1" license even though you have all IP Addresses for /20 subnet.
let me know if that helps.
thanks,
Michal