Add multiple aborption coefficients into virtual impulse response
A few points:


This is very poor model to approximate a real room impulse response, so you you should make sure that it can meet your needs before putting work into it. I think this one here may be already better: https://www.mathworks.com/matlabcentral/fileexchange/25965-fast-simulation-of-acoustic-room-impulse-responses--image-source-method-
The notion of a frequency independent reverb time (or absorption) is seriously flawed. In order to get something remotely realistic, you need to bake in some frequency dependency. Brute force would to be calculate one RIR for each octave, band-pass filter and summing  them up. 
If you want to expand this code, you should probably look at equation 9.


A quick hack could like like this

c = sqrt(r(1)*r(2)).^abs(e) .* 
    sqrt(r(3)*r(4)).^abs(f) .*    
    sqrt(r(5)*r(6)).^abs(g)


where $r$ is a vector with the reflection coefficients for each wall matching the x,y,z coefficients. That will be "close enough" for higher order reflection but pretty wrong for lower order reflections, especially if the reflections of opposite walls are fairly different.