First read the documentation located here:
http://openvpn.net/index.php/open-source/documentation.html
What you are wanting to do is indeed possible with the openvpn community software; however, it might require actual command line interaction with your DD-WRT router. The thing to remember about DD-WRT and any other OpenWRT derived system is that OpenVPN is Just Another Software Package to most of the people involved in packaging it up for distribution. I think DD-WRT has a pretty and somewhat useful configuration interface, but I have found it to be ... problematic with configurations that require more than what most people want from it.
At any rate, good luck. Please check back if you have questions after going through the documentation and how-to files.
For information on accessing the DD-WRT command line via telnet or ssh, please visit the DD-WRT wiki.
Regards,
Stephen