Yes, it can be done - as long as you have an existing cable router or a PC doing ICS with a spare network card or Hub/Switch. I've got cable with a NetScreen 5GT Firewall doing my routing for the LAN and I'm also using a DG834GT simply as a wireless access point.
All you need to do is set the DG834GT's IP address to something unused on the network, then disable its DHCP server so that wireless clients can get their IP from the DHCP server on the main router. After that you simply configure the wireless as you would normally - SSID and security.
For example, my 5GT's internal IP is 192.168.10.1 so I've set the 834GT to 192.168.10.2.
This way, the DG834GT is simply being used as a wireless access point and is performing no NAT or firewall duties of its own.