Basically to handle valves that are very far apart there are two solutions. One is to use a long extension cable to connect the main controller with the expansion board. This is suitable if you have lots of stations wired to the expansion board, allowing you to place the expansion board as close as possible to those valves. The second solution is to keep the expansion board close to the main controller, and use long wires to connect to valves. This is suitable if you just have a small number of valves connected to the expansion board.
Of course another potential solution is to get a second main controller, and use the upcoming remote station feature to have the first main controller talk to the second one. But this has more added cost.
There were a couple of requests of developing a remote expansion board with radio frequency chips. I don’t think there is sufficient demand to invest time and money on developing such a remote expansion board. Even if we did, the cost would not be much lower than getting a second main controller.