There is no reason to look for sb3 to zip conversion, simply because SB3 file is renamed ZIP archive. You can change the extension and/or extract the contents, but in order for it to work in Scratch, the project data need to be in ZIP/SB3 archive.
Also if you compile/export the .sb3 files to EXE/APP, the typical output is a .zip archive that contains the compiled file inside.