retrograde proccess question

Just adding yeast (assuming it's not a more attenuative strain than the original) would do nothing except increase the amount of yeast.

Adding sugar would, assuming there is still viable yeast in the beer, cause another fermentation, converting the sugar to CO2 and alcohol.

 

Yes. But if you plan to do this in the bottle, do some math, or you may make bottle bombs.

 

Yes, you´ll have some bombs in your cellar. Watch out for this, a friend of mine almost lost an eye for an explosive bottle.

 

Assuming you didn't do anything to kill it, there's probably already enough viable yeast to ferment any extra sugar you add, so there's really no need to add more. Though, aside from the extra sediment, it won't hurt.