What type of cable do you have? OpenSprinkler expansion board is shipped with a 2×4 pin cable. If you need to you can make a custom cable as long as it’s wired the same way. Even at the minimum it needs 6 wires, so your existing 4 wire cable is not sufficient.
I did ponder about the idea of remote expansion boards, but it’s not really cost effective. Because it needs to receive wireless signals and in turn control the shift register, it will need at least a microcontroller so in the end it will not be that much different from just getting a second main controller.