Unfortunately, Frank App currently only supports new digital ISIC cards purchased through Frank App. Unfortunately, the existing ISIC card cannot be combined with the new plastic card either. If you want the ISIC on the new plastic card as well, you can buy a new plastic card and add the new ISIC as part of the new student card when ordering.
If you want to use your existing ISIC number digitally, you can download the ISIC App on your mobile phone and order a new plastic card without the ISIC feature. You can register in the ISIC App with your own ISIC number. You can find the ISIC number either in the Frank App by clicking on the "Student Card Verification"-button on student card view, or on your plastic student card. The ISIC number starts and ends with a letter. You get the same benefits with a digital card as with a plastic one.