The first major problem you will have is splitting the signal. There is just no good way of doing it; tapping-in or piggy backing a wire without degrading the stereo imaging and signal. This is the same problem you will have hooking up a subwoofer without seperate outs on your pre-amp.
Things I've done to extend the top end of my speakers:
1. Cleaned them.
2. Replaced the strator panels.
3. Upgraded the spike feet.
4. Experimented with different materials under the speakers, preamp, amp, CD player, Turntable and so on - Note: Glass and Steel are very bright.
5. Upgraded interconnect and speaker wires to Nordost Blue Heaven.
6. Experimented with room acoustics and wall materials behind the speakers.
7. Noise line filters and power conditioners, cleaned connections.
8. Upgraded all electronics.
9. The list goes on - this is a great hobby, always room for improvement.
Martin Logan put the NAC drivers on the clarity and Aeon to try and compensate for the small electrostatic panels sweet spot. The large panels don’t need them. When Martin Logan added the NAC driver to the aeon they redesigned the crossover to accommodate the tweeter, it was not a simple soldering job.