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Yes, that's possible. You only have to hide the field (right click > hide field) or use hidden box widget in the form builder if you don't want users to see it or if you want it to be the admin field. Then, you can edit the hidden field when you edit the submissions in your form submissions page.
This guide explains it http://www.jotform.com/help/194-How-to-Add-a-Submissions-Status.
Hope this helps!