If .sb3 files are technically zip files, why do you need a converter? The issue is
The converter should check if the project.json is missing or malformed before attempting to create the SB3 file. It should warn you if there are missing asset references.
Are you looking to a modified project into the Scratch editor, or are you trying to export assets for use in a different game engine? Convert .zip to .sb3? - Discuss Scratch
When moving assets from other engines or older versions of Scratch, a converter ensures the file structure remains intact. How to Convert Zip to Sb3
If your zip file is actually an old Scratch 2.0 project (contains a project.json but the internal structure looks different, or the file extension was .sb2 originally), simply renaming it to .sb3 .