Use the Pitch block in advanced whammy mode. Set pitch bend amount to -1 semitone and set mix to 100% wet. Assign a modifier to the control parameter and set your expression pedal as the source. Since the whammy can only be set in 1 semitone increments, you'll have to limit the range of the control modifier by setting it's end value to 50% and it's mid value to 25%. That will give you a quarter step pitch drop at toe down, normal pitch at heel down, and a smooth transition between the two.