Perico, I think what you want to do is an Ethernet bridge. This is doable from Linux. We have small embedded processors at work that do this. What exactly you have to enable is kind of beyond me. However, you may want to dig around the DD-WRT forums. This port of linux deals with routers and I believe the folks there should know all the setup and config files to change.
Is the source code for the modification to the OpenSprinkler firmware available? I saw the compiled hex file in the zip file and the instructions mentioned that the firmware was changed. However, what exactly that was modified was not detailed.