You can definitely use mods outside of the ones on the steam workshop. In fact, after the last version update, when the mods in the workshop started to update, they often left messages saying "if you want this mod for an earlier build, you can find it on my github at … ". There’s a separate folder for manually downloaded mods.
I went through it quite a bit because I wanted to finish my current game before updating. It was a real chore manually disabling all of the auto-updated mods from the steam workshop since they broke my older version of the game until I removed them. I only re-added about 80% of the ones I had before the version update.