I can understand how helpful it is to resend your email to non-openers! I apologize, at this time there is not a way to schedule this feature after campaign is sent. Though it is a feature request our Product Team is collecting feedback on. In the meantime as a workaround you can copy the email and send it to a list of your 'Did Not Open's. You can also resend this email to a list of 'Did Not Open' contacts through our Quicksend feature.