Currently this is not an option through Workflow Rules or through the Milestone itself.
There is an Idea in the Idea Exchange where you can vote and add a post to the Idea at the link below:
Also in this Idea there is reference sample coding for Apex Triggers that can accomplish the automated milestones.
-Auto-completion of Case Milestones with triggers [Trigger on Case Comment] [Trigger on Email Message]
-Auto-completion Resolution Time Milestones with triggers
Note: Please contact you AE if you are not comfortable writing or implementing custom code. Your AE can get you a consult with a Professional Services to quote you the expected cost to complete your desired task.