It has never been tested, nor would it be supported. You wouldn't be able to do it with a Cisco VPN Client configuration (the vpngroup commands) on the PIX as this client does have some proprietary stuff in it that I'm sure the Fortress client doesn't do.
You may be able to get it working using the older CiscoSecure VPN client configuration in the PIX, as this was more of a standard client. Sample config for it is here:
http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a0080093f6a.shtml
As I said though, it's never been tested nor will it be supported if you call the TAC.
