Steam puts workshop items in the following folder by default: "C:\Program Files (x86)\Steam\steamapps\workshop\<appid>". The "appid" of The Quest is 428880 so you'll find all workshop items here: "C:\Program Files (x86)\Steam\steamapps\workshop\428880". If you only bought Thor's Hammer, there will be only one subfolder. In the/each subfolder there will be a "world.pak". That's the expansion pak. If you only have one, you can use as is, otherwise you can rename them to whatever you like so you are able to copy them into the same folder (C:\Users\<username>\Documents\The Quest\expansions".

I know it's not the most intuitive but it's an option for the more adventurous. ;)