The first RFID receiver that XTension supported was This serial model from Parallax Which also comes in a USB version that should work also after verifying that you have the proper drivers installed. It’s just got the USB/Serial adaptor chip on the board so it should work fine.
Since it’s designed to work with their microprocessors and not necessarily a regular computer like the one you’re running XTension on it doesn’t have a regular RS232 serial, but uses TTL level serial data. Thats low voltage serial basically. Slow speed regular serial you can extend over cat-5 cables for long distances without any trouble but it may be problematic to do so with a TTL level signal. I do not recommend using active USB extenders to get the adaptor closer to wherever you want to place the reader. Instead either use a Wiznet card and connect it off your wired ethernet network or use a regular USB/Serial adaptor at the computer, extend the regular voltage serial over the cables and then place a level shifter like this one near the device itself. Though a Wiznet card outputs standard RS232 levels over a DB9 plug the TTL level serial is available on the board at some solder pads that you can easily tap without doing anything difficult to modify the board.
Once you have the serial connection to XTension it’s really just a matter of holding the cards or fobs up to the reader to get the ID of them, then create a Unit in XTension with that number as the address. That unit will turn on whenever the tag is within range of the sensor and you can perform any action upon it in the units ON script as you wish. Open a door strike, Open a garage door, turn on or off an alarm. Whatever.