Upvote:20
Simple: go to any pharmacy in Germany, and they can convert your non-EU proof of vaccination into a EU COVID pass QR code. This is a free service and most any pharmacy is happy to oblige if they're not slammed with actual customers.
This QR code can then either be printed and shown when needed (you don't actually need the app), or scanned in with any EU pass application, although there may be some hoops to jump through (eg. Germany's own app is region locked).
More details, oddly enough, at this US Army site: https://www.army.mil/article/247734/optional_how_to_get_the_eu_covid_vax_digital_certificate_in_germany