yes this can be done. This is all set up in the Callmanager, just add the lines to the IP Communicator device, it should say that it is sharing once it's added. I have my DN and Ext. on a couple 7940s and also on two IP Communicators. I bounce around between my office and the server room all day long and don't feel like forwarding calls back and forth.
You may have to play around with connecting through the VPN for the best quality.