To the best of my knowledge the only route currently available to have WD collect data from an RFXCOM reciever is to connect the RFXCOM to Meteohub, then have WD collect data from Meteohub.
That’s the arrangement that I use.
Here’s where to find out about Meteohub:
http://www.meteohub.de/joomla/index.php
Here’s a topic describing how to have WD collect data from Meteohub.
http://discourse.weather-watch.com/t/33441