You can configure the Gateway Server to forward certain DevAddr blocks to the Network Server of your local cluster and forward (the same or) other DevAddr blocks to Packet Broker. See the configuration reference of the Gateway Server for more information.
You can also configure the Network Server to issue DevAddrs from certain blocks. See configuration reference of the Network Server for more information.
For what you want, that configuration (in yaml) would look a bit like this:
# ...
gs:
forward:
cluster: [ "00000000/6" ]
packetbroker: [ "26000000/7" ]
# ...
ns:
net-id: "000000"
dev-addr-prefixes: [ "00000000/6" ]
# ...