I agree with sakos that having two power adapters operating the same valve can lead to the risk of shorting the two adapters: the issue happens if both OpenSprinkler and your manual switch are switching on the valve at the same time, that’s equivalent to connect the two transformers wire to wire (i.e. in parallel). If they are synchronized in phase, this is fine, but if the phase is reversed, it will burn the transformers. However, if you can guarantee that they won’t turn on the same valve at the same time, it should be ok.
The way OpenSprinkler working internally is that one wire of 24VAC transformer is connected to the COM (common) wire, and the other goes to circuit ground. Each valve has one wire connected to the COM wire, and the other wire connected to a triac. The triac acts as a switch to ground. So when the triac is on, it directs the second wire of the valve to ground, thus completing the circuit and the valve turns on.
If you just want to add a manual switch to a station, you can add a switch between the station terminal to circuit ground (on the orange terminal block the wire closer to the USB port is circuit ground). That way, when the switch is on, it bypasses the triac and turns on the valve. This will be pretty easy and safe as there is no second transformer involved.