You can link the table to itself with a Cartesina self-join, and then use the script step Set Next Serial Value to update the serial number to the current Max + 1.
However: numbers are cheap. FM can look after all the serials and ID links they are used for. And it doesn't get upset about series not being complete. Why would you want to re-use the number and run the risk that somewhere in the child records that were created, that Parent ID was used, and now when you create that ID again the new record will have instant children that are NOT connected to it. Or 'are, but shouldn't be'.