1.  Go to Constituents profile of the constituent in question
  2.  Under memberships, click their membership's hyperlink.
  3.  Click "Renew now"
  4.  Change the expiration date back to what it was originally (when the membership was purchased without the add-on).
  5.  Change membership amount to $0.00 so as not to charge them for the membership again. 
  6.  Add the add-on
  7.  Enter payment information and save

If you would like to see the ability to sell a membership add-on outside of the renewal window added to Altru, please consider liking this idea in our community: Sell Membership Add-On without having to Renew/Upgrade Membership