  An extension of the Kadomtsev-Petviashvili (KP) hierarchy defined via scalar
pseudo-differential operators was studied in [16, 20]. In this paper, we
represent the extended KP hierarchy into the form of bilinear equation of
(adjoint) Baker-Akhiezer functions, and construct its additional symmetries. As
a byproduct, we derive the Virasoro symmetries for the constrained KP
hierarchies.
