  Observation of efficient Breit-Wheeler electron-positron pair creation is one
of the main goals for modern-day strong laser-particle experiments. We propose
exploring this process by colliding plasma-generated harmonics, from currently
available laser pulses, with a beam of GeV photons. We show that the creation
yield is enhanced for more than one order of magnitude, compared with the yield
obtained without high-harmonic generation. The robustness of the yield
enhancement is demonstrated by considering multiple interaction parameters and
the potential photon source for the upcoming experiments. Moreover, we show
that due to the evident field asymmetry in the plasma-generated harmonics, the
spin-polarization of the created pairs could be as high as $60\%$.
