very simple answer I am afraid - Boeing has done the full development and design in house on the MD80SF Freighter Conversion some years ago utilising the same door mount and structure as the DC9 and aimed at all MD80's but primarily the MD82 - however I am led to believe Boeing will not pursue the programme and/or offer/provide support to third parties who may wish to pursue a similar STC
I will not expand as to my own thoughts as to their reasons as it would not be correct to do so and I am sure commercial and rationalisation of fleet support comes into it