In this paper, we construct the bilinear identities for the wave functions of an extended
Kadomtsev-Petviashvili (KP) hierarchy, which is the KP hierarchy with particular extended flows.
By introducing an auxiliary parameter, whose flow corresponds to the so-called squared
eigenfunction symmetry of KP hierarchy, we find the tau-function for this extended KP hierarchy.
It is shown that the bilinear identities will generate all the Hirota's bilinear equations for the
zero-curvature forms of the extended KP hierarchy, which includes two types of KP equation with
self-consistent sources (KPSCS). It seems that the Hirota's bilinear equations obtained in this
paper for KPSCS are in a simpler form by comparing with the results by X.B. Hu and H.Y. Wang.