Better late than never, right?
I just found out this today.
Workflow file with a string argument
Right click → go to Activity Layout
Here you can choose the argument and what type it should be, one of which is dropdown
Pick the values for the dropdown
Publish it and add it under manage packages in a new workflow
and voila a dropdown.
This is a little old forum post, maybe this was not possible when it was posted.
I just found this out today, hopefully it can help somebody else 