You need a wireless ethernet bridge (aka, client bridge, gaming adapter). Some routers support reconfiguration as a wireless ethernet bridge w/ their stock firmware, while others (as Emerald suggests) would require third party firmware (e.g., dd-wrt, tomato). But the latter may be a bit overwhelming for some users. For them, it’s probably easier to just buy a suitable standalone device.http://www.newegg.com/Product/Product.aspx?Item=N82E16833122432
The above is just an example, not necessarily a recommendation. Just so you know what to look for.