Basic syntax conversion would be easy from VB6, but not the API calls, the libraries, the user controls, the error treatment, the calls to external dlls or ocx ...
Moreover, the architecture or a VB6 program, use to be some limited, with no real OOP. Then, even if you would success on conversion, you would have a mess of code with probably no options for maintaining.
I think a tool for conversion could be useful, but not for a whole project. Just for saving time on codify some rutines o modules (with simple algorithms) or some forms or dialogs.