Sure, you can add a retail router. To start, do you have a static IP? If so, you can buy any wireless router, configure its WAN with the supplied IP information, connect it to an ethernet port on the gateway, then browse to http://10.1.10.1 and disable the "Private WiFi" if your Comcast gateway has wireless capabilities (presently, only the Cisco DPC3939B model is.)
If you DON'T have a static IP, you can call 1-800-391-3000, speak to technical support and ask them to put the Comcast gateway into "bridge mode". Then, configure your purchased wireless router with DHCP on its WAN (most retail routers are pre-configured this way.)
