Yeah we had this discussion at work like 2 weeks ago. Here was our interpretation. Vesys is just looking for a part number on a wire it isn't supposed to take it to that next level of defining how those parts are made. Thats what spec drawing are for or a note. So setup the multicore as normal and give your harness supplier a spec page for the "in-house part" The reason behind this is because when you define a length in harness you are already taking in the twist of the wire into account.
At least that was our interpretation