I have had my DJI M3P for over a year now and insured it with Coverdrone, my question is can I purchase DJI Refresh or is it just for new purchases?

I don't know if there is a time limit but on mine that was 6 month old was possible. You just have to send them a video to proof that is fully functioning.  

If ok, they will send you a link by email. After this you have 72 hours to buy dji refresh.

https://support.dji.com/help/content?customId=en-us03400007411&spaceId=34&re=HK&lang=en

Hello there. Yes, you can get the Care Refresh when the product is brand new and unactivated or has been activated within 48 hours. As mentioned by Fyase, if the conditions are not met, please apply for online video verification of your product. After approval, you can purchase DJI Care Refresh within 72 hours of receiving the purchase notification. Have a nice day.