iRule to modify part of IP::client_addr

I'm trying to use an iRule to modify the client_addr to use as the snat.

client IP = 10.10.10.201 desired NAT = 192.168.1.201

Something like:

if { [matchclass [IP::client_addr] equals NAT_Clients]} {

      snat using this source address
     snat 192.168.1.$current_host_ip_last_octet

How would one go about doing that? using set and concat?