You mean vertical copper thickness of the track or it’s horizontal width?
If width is what you’re talking about you might have to create that part of the track as a footprint and then use no-fill-zones to keep copper fill to encroach too close to that structure.
Try a search of this forum with ‘microstrip’, ‘microwave’, ‘GHz’ or ‘antenna’.
‘metal lines’ meaning copper track?
If so, there was a modification to KiCAD v4 some time ago that had this, but it hasn’t made its way to the mainstream release yet.
Workaround is to use scripting and short straight pieces of track to create those areas.
Try a search of this forum with ‘copper arc’, ‘copper circles’ or ‘python scripting’.