Which accessory can act as an antenna for wired router to enable wi-fi or convert the wired router into wi-fi router? How is it configured? Please provide an ebay.in link.
As Bruce says, what you need is a Wireless Access Point. There are plenty of models available, but there are also many wireless routers which can be used in this mode, and many of them are cheap. When you compare prices, consider the possibility of replacing your router with a Wireless one, which will give you a much simpler "all in one" solution, and may cost you no more than an access point.
A wired router cannot be simply turned into a wifi capable device since it does not contain a radio. You need to be looking for a wireless access point (WAP) to add to your network which would then allow wireless devices to access your network.
This is the type of device you should be looking for as it supports B, G and N wireless networks and uses WPA2 to secure it (WEP and WPA are less secure).