Hello,
Welcome to the Microsoft Community.
Additional warranties for Surface are usually added at the time of purchase of the Surface or some time after purchase.
If your Surface Studio 2 has been purchased almost a year ago, we recommend that you visit Microsoft's official website - My Devices and see if you still have the option to sing the warranty on the information page of this surface, if it is there then you can purchase the warranty, if it's not shown that means that it's no longer available for purchase.
If there are any questions about this, you can visit the following link:
Microsoft Complete - Protection Plan for your Surface or Xbox- Microsoft
Check options 2 and 3 in the screenshot to see about additional warranties.
Best regards,
Huy-K | Microsoft Community Support Specialist