You may be able to achieve something using extensibility but it would be very clunky. You would be better of using a dedicated 3rd party ipam solution, for example InfoBlox or PHPIPAM, as they will scan subnets and reserve addresses that are found privately assigned within a scope. Most IPAM solutions can be integrated into vRA with either a plugin OotB or by writing an interface for it.
If you found this helpful, please consider awarding some points