Screenshot the other app first
There is no direct link to Stocard, Klarna, Supercards or Apple and Google Wallet, so start by taking a clear screenshot of each card in your old app, with the barcode fully visible. Save them all to your photos.
Run the import
Tap the plus button and choose Import from screenshots, then Select screenshots and pick the images you saved. ShowCards reads the barcode from each one and matches the brand automatically.
Anything it could not match shows a Choose brand button so you can set it yourself; check the numbers, then tap Import to add them all at once.
Duplicates are skipped
A card you already have is marked Already in ShowCards and left out of the import, so it is safe to include screenshots of cards you have added before.