There are a few ways to import stickers to GoodNotes, but I recommend this process, which involves downloading the stickers to your camera roll.
Download the files
Download straight to your device, or download on your computer, email/airdrop the files to yourself and open the email on your device and download to the Files app.
Unzip the Folder
In the Files app, unzip file folder by tapping on the compressed folder. Wait a few moments for the folder to be created containing the files.
You’ll find folders for Frames, Inspirational quotes, Headers, and more. Tap on one of these folders.
Select all the files
At the top right, tap “Select” and then on the left hit, “Select all”