That would be epic! I too would like to know this. I'm trying to bring objects over into the GM palette and for the life of me I cannot figure out how to convert a wall object to a scenery object
Having a resource like you want to create as a method of fast tracking maps would be excellent. I can see it being super useful for veteran and novice map builders alike.
Doesn't help me with trying to get prefabs into GM mode, but I applaud your idea none-the-less
So I am by NOOOOO means an expert, but playing around a bit in the editor I think the answer lies in "Level Templates"
To make a level template you select all of your items (just the parts you want so I would say avoid the terrain obviously)
With them all selected right click and choose "export to a level template". This actually will save them as a level, but that "level" (ie your building) can then be brought back into any other level.
When you drag a level template into a level you get a bounding box you can slide around before clicking to place down the building or whatever. The editor will treat it as a solid object, but you can right click on it and choose "Explode Level" to break it apart into individual objects again.
The problem is I'm unclear how you see level templates you have created in the root templates tab... I can drag and drop the Larian created ones into my levels, but no idea how to access the ones i've exported (even though they do appear in the level browser when loading or creating a new level.
You'll have to play with that a bit I suppose unless someone can explain to us how to properly use level templates.
I also hope someone has an idea of how to convert prefab objects (or level templates) in scenery objects. I really want to get walls and houses into the GM mode palettes.