Downward firing subwoofer

rickregal, you would have to remove both units, calculate the volume of the enclosure and find two subwoofer drivers that would operate together in that volume and fit the holes you have. Not likely. You might as well start all over.

@mijostyn sounds to complicated, I'll just and another downward firing enclosure to the mix. Thanks for the feed back.

Nothing wrong with a subwoofer with a passive radiator if it is designed well.  
The idea is to be someplace between a ported sub and a sealed sub.  
You get a somewhat improved efficiency of a ported sub with the extension of a sealed sub, and avoid the peaks that many ported subs have.
they also tend to be easier to integrate.  
in your case with the kicker the enclosure is too small for another active sub  
try to increase amplifier power if you can if you want more bass.  

Think of the passive radiator as an alternative to a port.

The passive radiator and driver work together with the volume of the cabinet.  It's a system and you should not attempt to alter without a proper understanding (aka speaker analysis) of where you are starting and where you are going. 

If you are getting an itch to DIY, you'd be better off building a  new subwoofer from a kit. :) Lots of good options at Parts Express.

The higher the Q rating of the Passive the easier it responds to the active driver. So a light cone, return spring and surround would add to a higher Q (Quality). Those types of passive are to blend the lower and upper octave of the driver. In other words they are more sensitive to MASS weight change.

In a heavy construction PR it may take ounces to change the Q. I suspect your current PR and driver are matched to hit a certain LOW point then roll off quickly. That is a good reason to use a PR,

In one with higher sensitivity (Q) it may only take a thumbnail of soft putty to change the Q and blend the upper and lower ranges of your sub..
Its a talent but well worth the effort to mechanically tune YOUR subs to YOUR main speakers to your room and YOUR amps..

THAT PR can do just that....:-)

It can also remove the challenge of tuning correctly to remove boom (too much mass on the PR) OR not enough mass to hit the bottom octave at -3-6db. That is the difference in a port vs adj PRs vs closed port IB design. You can hit 20hz at +1db vs 20hz at -6db via DSP and EQ. A 100 watt plate amp vs a 1000 watt plate amp..

If you know what your doing. It actually cost less to drive that kind of setup to.. A LOT less amp as long as they have 100@8 ohms or so on the dampening if your a real BASS head.. 100 watts a 8 ohms for plate amp will rattle the house with the PRs tuned.... Oh yea!!