Such a thing would be impossible... Even if they were to get over the practically-dead-weight excuse for an immune system, a quarian female would be biologically and genetically incapable of bearing human children, and human sperm wouldn't produce quarian offspring.



The reason we can reproduce with humans but not something like chimpanzees is because, while we are closely related taxonomically, we aren't even the same genus, that being the Homo genus, not to mention that quarians evolved completely separate from humanity.



If quarians were to somehow be taxonomically very similar to humans, even at least genus-wise, then such a thing would be possible, as we know that Homo Sapiens Sapiens has reproduced with other species and subspecies in the genus, the most noteworthy example being Homo Neanderthalensis, better known as The Neanderthals, that's how those of us who are of European descent have more Neanderthal DNA than say, those of us of african descent.



And yes, I get this is just a picture meant to be cute and that I'm ruining it by being nitpicky, and for all intents and purposes, I suppose I am being nitpicky, but, well, more scientifally puritanical, I just see a scientific inconsistency or impossibility and see that the actual universe is supposed to be more scientific or whatever, then it nags at me in the back of my brain.



TL;DR We didn't evolve alongside the quarians, I'm a scientific puritan at times, and I love to ruin the mood.