NAT over VPN

Hello guys,

I have two sites, site A and datacenter site B.  In site A I have 30 internal subnets.  In site B only one.  I have successfully created an IPSEC VPN tunnel between site A and B without NAT, so everytime I create a new department on site A, I need to ask site B to add the new subnet.

I would like to change the VPN setup in a way that all the traffic from all the different departments of site A is translated to the IP of our firewall so on site B, they only need to allow traffic from our firewall ip.   

In the VPN settings tab, I can activate NAT.  When I want to NAT department1 to the firewall ip, this works perfect.  When I want to NAT department2, the firewall ip is gone from the checklist because it is linked to department1.  

How can I manage to NAT all the departments to the IP of the firewall?

Thanks!