Hi Ajaz,
To add a new clause follow the below procedure, I have included the link to provide more clarity around the entire acl command process
Usage Guidelines
To add a new clause to an existing and applied ACL, reapply the ACL to the circuit with the apply circuit command.
To apply any changes to an existing clause on an existing and applied ACL, you must remove the ACL from the circuit with the (config-acl) removecommand, and then reapply the ACL to the circuit.
To remove a clause currently in use, you must remove its applied ACL from the circuit, delete the clause, and then reapply the ACL to the circuit.
http://www.cisco.com/en/US/partner/docs/app_ntwk_services/data_center_app_services/css11500series/v8.20/command/reference/CmdAclC.html
I would strongly suggest you have console access when using ACLs on the CSS as it can be a bit unforgiving when modifying ACLs
cheers,
Chris