Normally I would check the Workflow Settings page. If it's a reusable, then there should be a disabled Content Types setting showing which content type the workflow is intended for. And reusable workflows are created via the same Nintex menu where you also create Site workflows..
I've checked and can't see the Content Types setting. What I can see is below. I clicked on Association Columns, added a Test column and it showed up in the Set Drop down in the Set Field Value action.
Yes, but that will add a new column to your list as well. Unless that is what you want. And what if you try to use Update Item instead? Using the list and filter on ID = Lookup/Current Item/ID? would that allow you to see the fields?