Adding Field Level Security

Is it possible to add Field Level Security to a field after it has been created? I can return to a field I have created and add Field Level Security at a later point, but I cannot add Field Level Security to a field I did not create. Will the creator of these fields be able to add Field Level Security or is there another reason I can’t do this.

This is regarding an On Premise installation of CRM 2011.

Thank you for your help.