You can link an existing provider to an existing user within the User page. 

  1. Click on the Users tab.
  2. Select the User you want to link the provider to.
  3. Click on Provider Accounts on the left.
  4. Click on Add Provider Type
  5. You will be prompted to Link to all organizations or Only link to existing organizations. Choose Link to all organizations if you want the user to link to all the organizations the provider is linked to. Choose Only link to existing organizations if you want the provider account to only link to the organizations the user has access to. 
  6. Click on Save.

If the provider had patients, those patients will be connected to the user. You can also add patients to the user once the user has at least one Provider Account.