iRule for Snat pool association

Yes this is most certainly possible. Myself I'm awfully partial to using Data Groups, so if you create a Data group containing the subnets you need to snat, let's call it snatted_subnets and then you could use the following iRule:

when CLIENT_ACCEPTED {
   if {class match [IP::client_addr] equals snatted_subnets]}[
      snatpool your_snatpool_name
   }
}