You can either have the pools defined
on the AAA server and downloaded to
the NAS:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fdial_c/fnsprt11/dafprusr.htm#xtocid3
Or you can have the pools defined on
the NAS, and just download the name of
the pool for each user from AAA using
the Cisco-Avpair = "ip:addr-pool=POOL_NAME".