Board Thread:Questions and Answers/@comment-25881756-20141217000550/@comment-28525425-20160522173738

They may be lost souls, but once they are "slain" by a shinigami, those souls go to either the Soul Society (where they can then breed) or Hell (possible reproduction, though unlikely). If a soul can reproduce with another in the Soul Society, it would stand to reason that those same souls can reproduce as hollows provided they can find another hollow that wants to reproduce rather than fight and devour the other.