conversion of CSS SNAT to F5 LTM

You should be able to achieve it with iRule, as long as the pool members know how to get to the SNAT IP.

An untested example:

when CLIENT_ACCEPTED {
if { ([IP::addr [IP::client_addr] equals 168.108.164.0/24]) and ([IP::addr [IP::server_addr] equals 168.108.161.0/24]) } {
snat 168.108.161.147
}
}