Add multiple subnets as pool memebrs

A pool member is going to be a specific IP and a specific port. Can you elaborate on what you're trying to do?

yes, a pool doesn't have to be a collection of servers from a single subnet. As long as BIG-IP can route to them, they can be from any subnet. But as Kevin suggests, you can't just specify a subnet as a pool member, the BIG-IP defines a pool member as an IP:port combination.

OK..

Application will round-robin the services across many IP's in the subnets.

We don’t know specific IPs to add as pool members, so is there any way use entire subnet under the pool configuration[by using iurle?]

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.

We know the subnets and ports and application will move often with in the range of subnets.

How often? Sounds like a moving target. Is there any indication of the movement? Any chance that the service could alert the F5 of its new (and old) IP/port via iControl call?