I think the original post was just trying to take one user from a User List and put that user ID into antother user tpye field. This is easy, unless there are more than one users in the user list. In that case, you have to somehow decide which one to select and add to the single user field, and I can't help with that.
If all you want to do is update a List - User field with another user, than just add the user id into that value in the Pipeline, and it will update the single user.
My problem was that I was trying to iterate over a loop of users in a List User field, and then access the email property for each user to make a list of the email addresses for the List - User field. I ended up solving this by using a Formula - Text field to just list the user emails for me, and then I access that field in the Pipeline using Jinja.