Hey there,
Been doing this myself for several months now and adding assets to the game via replacement means, not with the CK. One of the biggest issues that people are missing is that there is a texture format issue between the two games. Fallout 4 is only partly PBR, while Fallout 76 is full PBR rendering. The Normal maps are not compatible, but can be fixed by simply changing the blue channel in the normalmap from white to black. If you don't, the object in the game will be very dark. The main issue is the specmap, those have to be totally remade. You can sort of take what is there and make some sweeping changes to them to get them to work, but its get pretty involved. You may need to play around with them. IT could be possible that the CK is freaking out over the textures. So make sure you make the right changes and save them in a fully Fallout 4 friendly format. Don't simply copy the textures from Fallout 76 and expect them to work. Someone made a mod here on the site where they ported over some assets and they did not do the conversion of the textures and things looked really off in the game. But once you convert a F76 nif to OBJ, any of the old data is wiped from it, and you are starting from scratch, and so you have to treat the nif likewise. I could try and walk you through converting the specmaps, and the way i have been doing it. I can show you images of all shorts of things that i have converted from the game as proof that getting the textures right does make a difference. I did have crashes in game when using stock F76 textures, so I hope this helps. Oh, and don't use Outfit Studio to make static object nifs, its adds body weighting data into the nif that static objects don't use, and you can't just delete the data in Nifscopee, it wrecks the nif. You might have to resort to using 3DSMAX, as I do, or Blender as many others do to get things working. The 3rd party nif tool for 3DSMAX 2012 through 2018 works very well, has a full installer, and works with Fallout 4, Skyrim SE, and all the older games nif formats. It does not support F76, so you have to do the NIfscope OBJ export first. Everyone will give you a different story as why you should use this app over that app, but the fact is, BGS uses 3DSMAX to make most of the nifs for the games, so its a good idea to use the same tool.