Agreed. I did the same thing like John last year. Even using freakz as a reference, I spent 3 month to finish a MAC/NWK/APS proto stack. Never trust your boss when he knows nothing about what a Z-Stack is! From John's post, I don't think he is familiar with ZigBee specs. From my own experience, reading and understanding spec will take a lot of time.
Porting should be much easier than developping from a scratch.