Sean is right: in firmware 2.1.3, each station has an attribute called ‘Activate Relay’. You can also set the relay pulsing time in ‘Edit Options’ — the idea is: when relay pulsing is 0, the relay will stay on as long as the station is on; when relay pulsing is a positive number, it will pulse when the station turns on or off (to simulate button presses).
Support for relay has been removed in the upcoming firmware 2.1.4 — primarily because the relay has been removed from all current and future versions of OpenSprinkler hardware.