From DD-WRT Wiki
When the WRT is used as an Access Point (AP) or repeater (WDS), the WAN port can easily be configured as a normal LAN port.
 Adding WAN Port to LAN Switch to use as an additional, normal LAN port
Note: Steps are true for firmwares current as of the author date of this article. v2.4, post SP2 firmwares. These steps were done with build 12307 (2009 06 17), widely regarded as stable. Other firmware builds of DD-WRT have the same or very similar steps.
- Open up the Web Interface to your DD-WRT Device
- Set WAN 'Connection Type' to Disabled.
- Down the Basic Setup page, the option WAN Port appears. Check the box 'Assign WAN Port to Switch'.
The WAN port is now the same as the other LAN ports.