humble minion
Legend
2. Muls are a crossbread between human and dwarves, a thing normally not possible in other settings, and muls are sterile like the namegiving horse/donkey crossbreed, so even if possible, there is no point in it.
With halfgiants it is even stated that their origins are because of magical help, since it would not work out the normal way.
But every race is stronger in DS so evolution parallel to the breeding of special slave races already did what you propagated. It is just muls not only are stronger, they also need less water than other races so in many circumstances they are more optimized. Being a bit dull they also do not tend to rebell as easy, as long as the basic needs are covered.
Could muls be created by means of reincarnation? In the real life the male mules are steriles, but the females can breed, but only with one of the two species.
I think Dark Sun would lose absolutely nothing of its essential character if the sterility and forced-breeding aspect of muls (and the name) was retconned out of existence, and they just became half-dwarves.