you can easily define DHCP pools in the Univention Management Console. For simple setups you actually don't need a pool, a subnet declaration often suffices, therefore the default installation doesn't create a pool. But you can create them.
Go to "Domain" → "LDAP directory", then navigate to your subnet declaration in the tree on the left. Click on that subnet declaration. Now you can add a new object of type "DHCP pool". See this screenshot:
As far as I know a DHCP relay agent includes the IP subnet for which the DHCP/BOOTP request was received (meaning: the DHCP relay agent has an IP address on an interface on which the DHCP/BOOTP request was received, and that interface's address/netmask will be sent to the DHCP server). Univention's DHCP server should then answer as long as there's a) a subnet object whose range includes the IP address and b) there's a "DHCP host" object, too. Note that I've never used DHCP relaying with Univention myself, therefore this is conjecture on my part.