I need to setup a Multinet on the lan side of a router I do not control

I am in a unique position of having to increase my network size behind a router I do not control. It is controlled out of our home office in Korea. IT also has a VPN setup in it for our other America facilities. The IT staff in Korea says it will not allow more than one subnet on the lan side. Our network is growing to the point I will need to add additional subnets. I need to setup another router with NAT behind it that supports additional subnets on the LAN side. Does anyone know a good router that will support multiple subnets on the LAN side? I was thinking of getting a Cisco ASA 5510. Will this work for what I am describing? Thanks in advance for your suggestions.