While your email is sent almost immediately after your workflow is triggered, it may not appear in under the Sent section in the workflow builder for a little while. You can also view the response information for campaigns associated to your workflows on you Response page by click Automated:
I hope this information helps! Please let me know if you have any questions!
I was reviewing this and think I may have misunderstood what your question, so I wanted to reach back out to you for clarification. Are you looking to add a previously sent mailing to a workflow, or were you perhaps looking to trigger a link-click workflow from a perviously sent mailing? If you're building a link-click triggered workflow, I suggest review the Notes at the bottom of this Resource Center article: