The populate with field gives you a list of pre-populated categories (Holidays, state names, months...etc) that you may want in a dropdown or multiple choice field. Your "current User List", should be amongst those to choose from. Or as a default reference item. Any changes made to your user list should auto reflect in the list. It doesn't make sense to export, then reimport users list anytime there is a user change. When the system already houses this data.