You can. I used that to subscribe for the old price until end of this year.Is there a way for a gamepass subscriber to buy months in advance ? Just thinking about stopping the sub vs buying multiple cheap months.
You need to first cancel your subscription, on the Microsoft website. Then have the gamepass App open on the side, and go to Settings, and click under manage on the subscription you currently have. It will open another screen, where you can then select "Extend". This button is only available here, not on the Microsoft site directly, and only after you cancelled your subscription. Selecting Extend on the app, sends you back to the Microsoft site, but now you can click Extend again here to start a payment (Paypal in my case).
You can then repeat this again and again, cancel subscription, go to extend (in the App), buy one more month, and so on.