Airport Express

I am trying to connect an airport express to an already established internet account through xfinity to expand the area of wireless.

The AirPort Express cannot connect to a third party wireless network using a wireless connection and "extend" or "repeat" the WiFi signal provided by a third party modem/router.

If this is what you are trying to do, the Express will not be able to meet your goals.

However, the Express can be configured to provide more wireless coverage if you are able to connect it to your Xfinity modem/router using a permanent wired Ethernet cable connection. The downside......you have to run the Ethernet cable from the location of the AirPort Express all the way back to the location of the Xfinity modem/router.

Yes, but you would not "plug" the AirPort Express into the AirPort Extreme since it would connect using only a WiFi connection.

The downside.......the last security update for the AirPorts was over 3 years ago, so they are falling quite a bit behind security wise.

Ideally, the AirPort Express would have a line-of-sight relationship with the AirPort Extreme since each wall or ceiling that the signal must pass through to reach the Express will slow the signal and signal strength by 15-20% on average.

The general rule of thumb would have the Express located about half way between the AirPort Extreme and the area where you need more wireless coverage. So, if the AirPort Extreme is on one side of the house and you need more signal at the other end of the house, you would locate the Express in the middle of the house.