Appworks Platform: How do I use the Assignment Building Block

I'm trying to assign entities to users, who will "own them". I am using the Assignment Building Block. I can make it work by using a property to store the user name and typing it manually upon creation. But I want to be able to select a user from a dropdown list. I have tried changing my property to be an enumerated text, and using Dynamic enumeration. That led me to trying to figure out how to create a BPM that can get all user names and return them in the form that the enumerated text object requires. I found the webservice that gets all users, but I have no idea what format the enumerated text object requires, nor how to shove the output into that format. I really feel like there should be an easier way to do what I want.