Log in to your Pocket account and go to the export page: https://getpocket.com/export. Export your data: you will receive a ZIP file containing one or more CSV files (the first of these files is named part_000000.csv).
From the user menu in the top right corner of the wallabag interface, click "Import" then choose "Pocket CSV" (available directly here). Upload your CSV files (one at a time).
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