Referencing BMF's diagram, if the MIDI pedal supports expression pedals, you'd connect the expression pedal to the MIDI Controller, then use CC to send it to each MIDI device downstream.
Alternatively, again using BMF's diagram, you'd connect the expresison pedal to the Factor Pedal (Time Source) and have it transmit CC to the MIDI Devices downstream.
Or, avoiding MIDI for the expression pedal, you can make a cable like the one described (with one more 1/4" TS plug than you have Factor Pedals) and attach one plug to each Factor Pedal's Expression Pedal Input and the remaining plug to the expression pedal itself.
Honestly, I think it might be easier and use less cable to just attach the expression pedal to either the MIDI Controller or the Factor Pedal (Time Source) and use MIDI CC, but any of the above should work.
