How to add comment on my submissions?

Is there a way to add a comment or note on a received submission, for internal use?

If I understood you correctly you want to add comment inside of form submission. Unfortunately that is not possible to achieve inside of form submissions.

Possible workaround would be to add Comment field inside of your form and hide it. That way it would not be shown inside of your form, but it would be visible inside of your excel report and then you can add your comments inside of excel report.

Inside of this article you can find out how to create excel report:

http://www.jotform.com/help/44-How-to-Export-Form-Data-to-Excel 

http://www.jotform.com/help/101-How-to-create-an-Excel-Report 

Hope this will help. Let us know if you need further assistance.