External pools can be used; but you need to specify option 118
in the NC profile's DHCP settings; it is used for IPv4 Subnet selection option.
<option number> <network-address for the ip-subnet> <type ip-address>
The option 118 informs the DHCP-server, the pool from which it should lease an IP. The option number might change; regarding the server software, 118 is used in ISC DHCP.