  The anomalous Hall conductivity of "dirty" ferromagnetic metals is dominated
by a Berry-phase contribution which is usually interpreted as an intrinsic
property of the Bloch electrons in the pristine crystal. In this work we
evaluate the geometric Hall current directly from the electronic ground state
with disorder, and then recast it as an integral over the crystalline Brillouin
zone. The integrand is a generalized k-space Berry curvature, obtained by
unfolding the Berry curvature from the small Brillouin zone of a large
supercell. Therein, disorder yields a net extrinsic Hall contribution, which we
argue is related to the elusive side-jump effect. As an example, we unfold the
first-principles Berry curvature of an Fe3Co ordered alloy from the original
fcc-lattice Brillouin zone onto a bcc-lattice zone with four times the volume.
Comparison with the virtual-crystal Berry curvature clearly reveals the
symmetry-breaking effects of the substitutional Co atoms.
