You are right, if something is not supported , it means that VMware didnt test it and certify it. That doesnt mean it wont work at all , you may always give a try and check if that works before going for alternate action.
It should work but didn't tested because of EOL of OS.
You should try with latest Converter and keep in mind that para-virtual OS could not be converted.
Since vCenter Converter version 6.0, it supports up to the Centos7 (64bit) and Centros 6.x (32bit and 64bit) versions.
Download VMware vCenter Converter Standalone 6.2: Download Center for VMware Products