as I could find in the Stadia helpcenter it looks like you can't stack the new Premiere code with your current Buddy Pass. I still feel it's not quite clear if you could add it when the Buddy Pass ran out. This might be worth to check out before the purchase with Stadia support.
"If you upgrade from Buddy and purchase a Premiere edition, you will receive a new Premiere code via email which you can give to a friend. The new Premiere code cannot be stacked upon your current Buddy Pass." Source: This article