They can but it's very expensive and very involved and not worth it. It requires removing the headliner, removing the side airbag (riveted), drilling holes in the roof, installing the rails, riveting the airbag back in, and reassembling. Plus the roof rail kit cost some stupid amount (hundreds more than the factory option). Edit: Heres the kit for $850. There's installation instructions at the bottom of the page: https://new.minimania.com/part/G3NMX1150-P/Mini-Cooper-Roof-Rail-Kit-Oem-Gen3-F55-F56
I wish I had a way to measure the wind noise. It's mainly noticeable at highway speeds - you can definitely hear them. But it does not bother me. In the winter I will go bare roof.