Hi Donald,
Assuming you are using ACS as your RADIUS server, you can set the static IP address in the "Assign static IP address" field in the user's account settings. You would then create a group-policy of type "external" to be used by all your VPN users. The ASA will send the username to ACS during Xauth and ACS will return the IP address to be assigned to the client.
Hope that helps.
-Mike