The option to purchase popped up in the Mimo app upon initial activation however not I cannot find a way to add this.  I activated my OM4 1 day ago so I am still in the window to add.  The DJI Store only has this available with a new purchase.

Hi there, please purchase the DJI Care Refresh 1-Year Plan (DJI OM 4) from this website: https://store.dji.com/product/dj ... ?from=menu_products, DJI Care Refresh can be bond to a handheld device activated within the last 30 days. Thanks.