irule translate ip address

Do you want to translate only client ip 192.168.1.1 or any client ip?

if you want to translate any client with source 20.20.20.20, you can create a snat pool with this IP address and assign it to the virtual server... no need an irule!

try this:

when CLIENT_ACCEPTED {
    if {[IP::addr [IP::remote_addr] equals "192.168.1.1"]} {
        snat 20.20.20.20
    }
}