Up until now I was assuming the game would come with a tool labeled along the lines of
AHIT Construction Set or something like that, which would enable players to design their mods, should they be so inclined. The games is advertised as having "full modding" here on GOG*, so it would still have to work, for users without a Steam account - like me. (Or in cases where the Steam website gets blacklisted by your country over night.) Also,
splitting their modding community would seem... counter intuitive.
In any case I would assume that there would be some documentation about how the process works in the menu where you load the mods.
* Community-funded, community-focused
A Hat in Time was successfully funded on Kickstarter, reaching nearly 10 times its goal! In order to celebrate our community, A Hat in Time is releasing with full modding, an in-game screenshot mode, and more!