Is it possible to create an edit link for submissions that have already been made prior to setting up the autoresponder email?

Yes it is possible to achieve your requirement and here are the options available:

#1. You can manually create the edit link of submission. The edit link URL uses submission id:

https://www.jotform.com/edit/SUBMISSION_ID 

You can grab the submission ID of the submission you have received and add it in the above URL by replacing "SUBMISSION_ID". Please check the screenshot below on how to get the submission ID:

#2. Alternatively, you can edit the auto-responder email alert of form, add "{edit_link} tag and enable "Send On Edit" setting. Please check the screenshot below:

You can then edit the submission you have received in your form and submit it. It will trigger auto-responder email and it will be sent to user with edit link.

The following guide should help you how to edit submissions you have received in your form:  https://www.jotform.com/help/325-How-to-Edit-Submissions-Made-on-Your-Forms 

Hope this helps.

Do get back to us if you have any questions.