Configure Sonicwall TZ-190 with Cisco VPN box

On the Sonicwall we have an assigned range of WAN IP addresses 123.123.123.1/255.255.255.248 while the WAN IP address we need to use is 111.111.111.229/255.255.255.252. This configuration has worked fine for our requirements until now.

x0:lan - 192.168.168.1/255.255.255.0
x1:wan - 111.111.111.229/255.255.255.252
x2:
x3:
x4:
x5:

We have an external US dept who want to introuduce a Cisco box-to-box VPN to allow users in our network to access servers in the US one. They have supplied a pre-configured Cisco ASA device configured with a public IP address 123.123.123.3 (i.e in the range of useable IP addresses we have).

To get this incorporated in the network I believe we need to add another PortShield interface in transparent mode to allocate the required IP address to this device and allow it to create the box to box VPN. However because of the disparity in the WAN IP address and the useable IP address I get "Error: Transparent Range not in WAN subnet" when I try to add this interface.

I have no access to the Cisco box so I cannot see a way to alter the setup to use NAT instead of transaprent mode and I'm not sure whether it would correctly build the VPN tunnel using NAT. Does anyone know if this is possible and if it is what I am missing?