Yes the rails are an option best to be ordered as is. It seems possible to put them on after, but a real pain in the butt to do it. As for what size rack/cross bars fits on it next.. no idea.
https://f48.bimmerpost.com/forums/sh....php?t=1582453
Another Edit.. I just remembered this gorgeous X2 Msport all done up and.. with cross bars installed.
https://f48.bimmerpost.com/forums/sh...ight=roof+rack