A couple of things to consider:
Adding a driver will affect the impedance loading on the amplifier, increasing or decreasing depending if wired in series or parallel.
The published (or expected) frequency response of a speaker is different than the in room response. A speaker's in room low end is often ten or fifteen dB higher than published specs taken in test chamber. So even if there was a way, realistically how much of a benefit would it be.
You could try a passive radiator or a down-firing port but that takes as much design horsepower as crossovers since you have to take the effect of the cabinet on the full range driver's performance.
Or, you can try a simple first order inductor.