Belief expansion is the operation of incorporating new information into a belief base or theory. If a belief state can draw on a set of fundamental beliefs, new pieces of information are often not simply added, but explicitly tied to fundamental beliefs by means of explanations. Abductive expansion is an AGM-style belief change operation that augments a theory with an underlying explanation by means of abduction. AGM-style belief change presupposes deductively closed theories and thus makes no distinction between the explanation and the explained. In this paper, we extend abductive expansion to belief bases, which do not presuppose deductive closure. Within bases, abduced information is introduced in response to unexplained input and can therefore be explicitly identified as a newly formed belief. We provide an axiomatic characterization along with constructive methods.