This question has been brought up quite a few times before. Here are two potential solutions:
Solution 1: Make a custom extension cable using an Ethernet cable or something similar. On OSPi, the extension connector has 8 pins, though 3 of them are all connected to ground (to effectively increase the current capacity of the ground wire). So at the very minimum you need 6 wires (using 1 instead of 3 ground wires). This way you can place the expansion board far away from the OSPi. Keep in mind that you also need the COM wire, which is not part of the zone extension cable.
The downside of Solution 1 is that since the extension cable carries logic-level signals, the resistance on the extension cable over long distance can impact the signal quality, and this may be prone to noise and signal interference.
Solution 2: Use an Ethernet cable or something similar as the solenoid cable (the conductor cable you mentioned). The advantage of this over solution 1 is that the cable does not carry logic signals so it’s less prone to noise and signal interference. However, you will need a lot of wires if you have a large number of stations on the other site. Of course this is based on the assumption that a long Ethernet cable is cheaper than the equivalent number of solenoid wires (which I believe is true).