1) It depends on which version of OpenSprinkler hardware you have:
– for OS 2.x and OSPi: the cable uses 8 wires
– for OS 3.x, if your controller is AC-powered, it only uses 6 wires; if DC-powered, 8 wires; if Latch, 10 wires.
Judging from your description, I assume you have OpenSprinkler 3.x. If your controller is AC-powered, then you only need 6 wires. A standard Ethernet cable has enough wires to make a custom cable.
2) I am not sure what’s the name of the plug, I know that it’s the same type as IDE cable in computer systems, but with less number of wires (IDE cable usually has 2×20 wires).
3) No, the expander does NOT need to be connected to a transformer. However, all zones on the exapnder share the same COM (common) wire, which is not part of the extension cable. So if you are thinking of having some zones 50m away from the main controller, and some more 100 meters, then those zones must also have a COM (common) wire, and that wire needs to go all the way back to the main controller. So technically your common wire also needs to be that long, in addition to the custom extension cable. On the other hand, the common wire is shared by all zones so you only need one such wire.