Set your project settings to UHD resolution, 3840x2160, import your 5.7 k footage into the project, then use either event pan crop or track motion to scale the clip up so that there are no longer black bars in the image and it fills the 16:9 frame. Track motion might be preferable, because then all the clips that you drop on that track get the scaling applied to them.
You interlace/field order settings also need to match, you want it to be progressive, and your frame rate should also match.
In VP20 cropping of video to project aspect ratio can be done automatically, just an option in the preferences and working the same way as automatically cropping a picture to project aspect ratio.
What is the easist method tot put 5,7 footage from a GH6 (17:9) in a UHD project (16:9))?
The question was how to import 5k7 17:9 footage into a UHD 16:9 project and not how to setup a UHDproject.
Due to the different aspect ratios the 17:9 footage will give black bars in a 16:9 project, the different resolution is no problem. Avoiding the black bars can be done automatically since Vegaspro20 (depending on preference setting) and also done manually by rightclicking in the pancrop tool and selecting "match output aspect ratio".