if you are using network connect pool , i do not think so, because the PCS will take the first available ip on the pool. if you are using a external DHCP server for ip allocation, you may setup rules on that.
else, but not good from "design" or administration pov, it is to setup a static mapping with a specific network connect ressources profile for each user
In addition to the radius/ldap attribute option this can also be achieved if you use an external dhcp server.
By default when the Connect secure device makes a DHCP request it embeds the clients hostname, you could change this to send the username (refer admin guide and search for "DHCP options" for config instructions) and then configure DHCP server to provide the same IP each time if the request contains the same username.