I don't follow what you want to accomplish. Could you outline the goal.
IP pools are used by dynamic vpn where computers connect into the firewall and then get assigned an address from this pool to access resources.
Site-to-site vpn connect agreed upon network segments on both sides of hte tunnel and permit the communication over the vpn link.
Are you looking to nat the vpn traffic? In that case you will need to create a route based vpn instead of a policy based one.
What is your connection scenario?