I tested if it is possible to define a zone and a subzone FRED. In my case a zone cr and a subzone ac.cr.

You can do that in this way:

1. Create the domain with a certain zone number, say 4 in our example. The subzone is created, say, with number 5. That is, 4=cr. 5=ac.cr. You must define records in the tables zone, zone_soa  and zone_ns.

2. Define stanzas for both in the genzone.conf file.

After this you can generate independent zone files for the zone and the subzone, say db.cr and db.ac.cr.

I hope this is useful.

Mario