I'd like to know if it's possible to make transparent caching with squid and
CISCO CSS 11000 device. I need to have the squid server making the HTTP
request using the client IP address as source (IP spoofing).

The equivalent for this feature with Cisco cache engine 500 is the command:
http l4-switch spoof-client-ip enable

Is there any way to configure this in squid.conf ?