Do I need iRule and SNAT?

So if I'm setting up one virtual server IP address to service two pools, should I set up different route domains so that I could specify the source port and address of the internal vs the external clients, or is this unnecessary? I'd like to be able to say: if the client is coming from anywhere except this internal subnet using https, translate this to your own ip and send it to the server listening on this port. I haven't found a way to specify what source port the SNAT should work on for translation, unless I'm just missing it somewhere?