Need to add multiple destination subnet in the virtual server(forwarding ip)

You cannot add multiple destinations for a virtual server but you could accomplish what you want a couple of ways. First, if there are a small number of destination subnets you could create one forwarding VS for each subnet. If that idea does not appeal to you then you will need an iRule similar to this: Access Control. Then create one forwarding VS with wildcard destination, 0.0.0.0 and the irule will restrict which destinations are accepted.