I had a look at this sometime ago, Webasto seem to make the roof systems as original equipment for a lot of manufactures but the Arona is not listed for aftermarket fitment. Probably something to do with the roof ridges.
Looking at the new VW T-Cross, which is their version of the Arona, on the German website, that does not seem to give the option of a sunroof at all and most VW cars do have that option.
Looks like Seat have missed out here as with the black headline it does make for a rather somber interior. There was an early post from this year from a member in Jersey saying the same.