A zip to apk conversion would essentially represent repacking of a ZIP archive to APK format used on Android, something that can mean several things.
First of all, it's worth mentioning that .apk files are .zip files that had their suffix renamed from zip to apk. The internal structure of the .apk is however set (standard), so you can't just take any random ZIP archive and turn that into a proper APK file simply renaming the extension.
You can find apps that claim that they can help you convert zip to apk, like zip2apk converter, but such programs quite often just rename the fles.
Updated: April 13, 2017