How can the sender sign in DocuSign once all the recipients have signed?

The best approach is to add yourself as the last Recipient by using Signing Order. If you did not add yourself prior to Sending the envelope you can try to use Correct and then add yourself as a Recipient, add fields associated to the new Recipient and finish correcting.

If you are unable to correct because the envelope is completed, then I would suggest using the "forward" option where you can add your signature under a new envelope and add any carbon copies as needed.