How to add an existing field to an user account? I created a vocabulary in Structure>Taxonomy>Add Vocabulary. Then I add terms to it. 

My Vocabulary fields look like this : 



I want to display this vocabulary into accounts fields, I want to add existing fields but I can't find the term reference.
You have to use "Add new field" until you haven't used your term reference field for the Departements at least once before. 

If have created this field once, you can reuse it for other content types.