Adding NAT to school router

Hello

Here in ireland primary schools get a router from the dept of education. The router is locked and only accessible by the tech guys behind the broadband helpdesk. The body that manages internet connections to schools has ran out of ip addresses and as such they cannot increase the numbers allocated to schools. They suggested to implement nat by attaching an external device to the dept of education supplied router. Although this router can support nat they chose not to support its implementation in order to have more ip address available. Can anyone point me in the right direction on how to set this up and what should i purchase? I suppose it would be a hardware firewall or a gateway of some sort. Any help will be greatly appreciated. Thanks in advance