@Gordon
The only way to do this is to connect a hub to the HDX101 and your devices - the PC and Xbox, say - into the hub.
Depending on the availability of power sockets in your home, you'd probably be better off just buying one or two more HDX101s.
Another option is Netgear's XE104, which is a single powerline adaptor that has four Ethernet ports on board. You can read Register Hardware's review of it here:
http://www.reghardware.co.uk/2007/07/16/review_netgear_xeps103/