Hi Xaf, the correct term is a "reducer hex nipple". Have a look below:
https://www.amazon.ae/s?k=reducer+hex+nipple&ref=nb_sb_noss
From the looks of it, this bolt is an M10. I think your thread in your roofrail is M12 but i am not sure.
If so, then you need a hex reduce nipple from M10 to M12.
A few months back i got some hydraulic hoses made by INFLEX, located in Al Quoz.
25.142727, 55.236064
These guys really should be able to point you in the right direction. I saw cabinets full with these things at their shop.