If your flow sensor comes with 2 wires, most likely it’s supported. You can just wire it to the rain sensor terminal, and go to Edit Options -> Weather and Sensors to select the sensor type as ‘Flow’.
If the flow sensor comes with 3 wires, you will have to check the datasheet / spec to find out whether it needs to be powered, and what type of contact it provides. It’s still possible to get it to work with OpenSprinkler, but you will need to find out how the flow sensor works first.
Technically, the sensor terminal on OpenSprinkler is designed to connect to a ‘switch-type’ sensor. All rain sensors and dry contact flow sensors are such type of sensors — they are essentially rain-activated (or flow-activated) switches. The sensing pin is pulled high normally, so when the sensor switch is open, the sensing pin reads a digital high, and when the sensor switch is closed, the sensing pin reads a digital low. If your sensor outputs an digital/analog signal (and usually this means needs to be powered), it can still work as long as the output voltage is within limit (should be no more than 5V).