Add multiple subnets as pool memebrs

You could potentially loop through a list of IPs with an iRule, but what guarantee is there that a service is listening on that IP? It makes no sense to send traffic to an IP address and port if there's not a server somewhere listening on that IP and port.

Is it that you don't know the listening IPs (and ports), or that they can change often?

If the former, you need to find out. If the latter, you may want to consider running a periodic script that "re-compiles" the pool member list based on the availability of IPs in a given subnet.