There's been success in converting some fairly advanced mods from fallout 3 to NV over at TTW. I'm wondering if that's unique.
On Skyrim, I'd made my own house/companion mods for my character.

I think I was playing as fallout 4's settlement builder via the creation kit. :p Anyway, as far as I know, it's possible to take the image format and change it to fallout's image format. But to re-create that house in the geck, I think I have to rebuilt it, re-placing everything piece by piece, flower by flower, plate by plate. Am I right, or is there a simpler way to get the skyrim house/companion esp info into fallout?

it's probably possible, you'd have to port over the Skyrim meshes and textures first off (updating the meshes using Nifskope if you do so), and if packing into the new .ba2 files make sure you use the archive thing that comes with FO4 GECK*; other than that, make a copy of your house mod (ie: My House (COPY).esp), open it in XEdit and remove "Skyrim.esm" as the master and then open it up in XEdit for FO4

now, your house mod's exterior will likely have to be redone entirely since it's from Skyrim and would be in a completely different game's "worldspace", but they've been using the same game engine for about a decade so you shouldn't encounter too many issues when porting. anyways, i'm not sure if this would be simpler than rebuilding, but it's definitely something that you could use as a learning experience if nothing else

IMPORTANT THINGS TO NOTE BTW:

Code:

- make backups of everything, trust me on this lol
- delete all navmesh information in the original file of your mod but ONLY in the copy of it, you will have to entirely remake the navmeshes otherwise i'm certain the CK would explode
- before porting over the file and removing "Skyrim.esm" as a master, make sure you check for errors in XEdit and fix any and all of them

if you're not experienced with the Creation Kit(s) and XEdit lemme know and i'll help!

*DDS files must be opened in a separate archive type