So, there is no signal in any other building?
Does your network consist of any type of Cisco switches, that may control traffic over every ethernet jack in the office (MAC Filtering per switch)? If so, ignore my suggestion next, as something else will need to be done…
If not, and your network closet consists of just basic 24-port switches, you can get away with putting a 4-port switch by a computer furthest away from the existing Wireless. Plug the switch into the wall jack where the computer was and then plug the computer and a new AP into the switch. Configure the new AP with the same SSID and security settings as the Zyxel wireless AP.
Make sense?