If nobody else know the answer I would say Buy. Try. Enjoy or Refund

I don't see it working, I don't see steam even letting you buy the season pass without the base game owned on steam.

this question comes up for a lot of games. I have never seen a game and DLC work if one is on Steam and the other is on a different service.

No its not going to work. Steam store adds purchases directly to your steam account, no ifs, no buts, no option to use them on any other platform.

So if you don't own the base game on your steam account you're not going to be able to play DLC you bought from steam.

They just added it to humble monthly; if there's other games in the package that are interesting then you end up with a drastically-reduced price for a Frost Punk steam key (+1 dlc).
I just started playing this last night on Game Pass but the humble monthly has both this and Pathfinder which I wanted so I guess I'll own FP on Steam now.

Doesn't seem like the files from the game pass version can be copypasted into the Steam version dir to avoid downloading the game again, if anyone else happened to be doing a game pass --> steam switch. The main resource file size doesn't match (probably because of the new DLC on steam which isn't(?) on game pass).

You can copy-paste your save from C:\Users\YOUR NAME\AppData\Local\Packages\11bitstudios.Frostpunk_gwy9gn5q9j1y6\LocalState to Steam's remote folder for the game (323190). I had to create a save in the steam version, then copy my save to the folder and change my real save name to the new name to get steam to recognise it.

what happens if is the other way around, steam base game and xbox games DLC on PC

I don't think the game and DLCs can see each other if they are installed on different services.