Putting firewall behind another router firewall/router for VLANs

G'day,

I have a client who has a Sonicwall NSA 4600. I thought they were going to bring in a new internet line, but it seems like the want to use their existing.

I need VLAN access to at least 1024 IPs so I would use the Sonicwall to do that, but can I put the Sonicwall behind their office firewall/router, and give the Sonicwall WAN IP an internal IP from their office firewall?

I suppose another option would be to try and get their ISP to provide them with another public IP and then attach the Sonicwall directly to that assignment.

Any thoughts? Thanks for your help!