Here's an couple posts regarding various ways to add a narrow band filter for CW.
Most of us would be happy with an add-on audio filter between the rig and the headphones.
That's by far the simplest solution.
Making the crystal IF filter narrow band would keep a strong nearby signal a few hundred hz away
from causing distortion in the demodulator and audio amps.
Simply adding a CW crystal filter in parallel with the SSB filter as
originally suggested by Farhan and implemented by PA1FOX seems ideal.
The two crystal filters are at tuned to much different frequencies, select one or the other
by reprogramming the BFO and local oscillator frequencies.
Lots of talk in the forum about using the Elecraft scheme to adjust the existing SSB filter
for a narrower CW bandwidth on the fly, I'm not aware of anybody who as done this
with good results.
Simply having two plug in crystal filters would be fine, would allow easy experimentation
with other filters.