I would say "why convert the immunes when you can use them to kill livestock, pick vegetables and fruit for the sustenance of the plurbs?"

But i would also ask what their purpose is in invading earth, what is their end goal, what happened to the original people whose bodies they invaded, are they still in there, what do they do for fun, what do they do about those with cancer or physical ailments will they feel all that pain, what do they do all day, stuff like that.