From Pinboard

Export your Pinboard data

On the backup (https://pinboard.in/settings/backup) page, click on "JSON" in the "Bookmarks" section. A JSON file will be downloaded (like pinboard_export).

Import your data into wallabag

Click on Import link in the menu, on Import contents in Pinboard section and then select your json file and upload it.

Your data will be imported. Data import can be a demanding process for your server.