Hello,
If you ask me that is it possible - well answer is yes
But, Adding rear disc brake is not a direct fitment like removing drum and adding disc in rear
It needs a comprehensive modification and trust i really doubt on manship of such a sort of modification, any such modification if gone bad can end up been fatal.
Also, can add up :- Having all 4 disc brakes improves braking effectiveness but not something which can be differentiated by a regular driver. In day to day life - a very very marginal difference can be felt in, and for all practical reasons been the front has disc - it makes no sense to alter in braking system.
I wont recommend
Hope should help !!