Well, we have never tested the main controller and expander separated that far away (i.e. more than a few meters away). Sure you can make a custom expander cable using an Ethernet cable, as long as you make sure the wiring of the cable is the same as the provided expander cable. The connector you linked to is fine, or you can use any 2×5, 2.54mm spacing connector. The advantage of the IDC connector is that it’s polarized so it prevents mistakes when plugging in the wrong way. The bigger issue with a long cable is that the signal quality can degrade over long distance. Also, don’t forget about the COM (common) wire — all zones connected to the expander must also have a common wire that should be combined with the common wire of zones on the main controller (or if it’s a separate wire, it can go into the section COM port on the main controller). The bottomline is that in addition to the expansion cable, you most likely also need a long common wire (though you can make use of one of the spare wires on the Ethernet cable as common wire).