Under the configurator, there are options for roof-mounted ski racks and bike racks. However, the attachments shown appear to be mounted to a 4-door and not a 3-door hatch. The European attachments appear to have mounted to the hatch instead. I presume the design has been modified to make roof mounting work properly?
I had noted the difference in racks in that in Europe, they sell a hatch mounted rack, here in the US, it's a Thule system and from what I understand, Thule and others that make roof racks make them for cars with and without gutters.
Thus I suspect that being from Thule, it's a standard item, sized to fit the 500 but perhaps others that are of the same or similar width (length of the cross mounts).
But as for the decision NOT to do the hatch design, I can't say but I was not totally enamored of that idea, rooftop, yes, but not hampering the rear view IMO.
Well, based on the mopar pics from the naias, it looks like the representation on fiatusa.com configurator were not accurate. Looks like it will be hatch-mounted like the euro car. Oh well.
The Thule 480 Traverse roof base will work with the 500, but it requires the 487 Short Roof Adapter. Fit kit is 1611. This is a pricey little puppy - over $400 when all is said and done, and that's before you put any actual carriers on the thing. Yikes. Not sure which US hatch mount carrier will work. The Thule European model numbers and configs are completely different from the North American ones. The 9003 looks interesting, and the bike carrying method of this model is a must for carbon fiber frames, but no way of telling whether it will work on the 500 (especially a Prima with the spoiler).
I put my road bike inside with no problem. If I wanted a rack, I would buy the receiver hitch from the Fiat Studio and then add either a custom fork mount rack made by a welder (surprisingly competitive), or one of these: