Hello,
as long as your DHCP address pool matches the source list of your NAT statement, you should be fine, since everything your DHCP server dishes out would be translated to the IP address assigned to your ISDN interface. Let´s say your DHCP address pool is 192.168.1.0/24. So all your users would get addresses from this address space. Your NAT statement would look like this:
ip nat inside source list 1 interface bri 0 overload
and the (access) list 1 mentioned in this statement would be:
access-list 1 permit 192.168.1.0 0.0.0.255
HTH,
Georg