To achieve alpha blending-like transparency an idea is to:
store alpha values in the voxels
use a multiplicative (1-alpha) accumulation when filtering the voxels and after encountering a voxel during light accumulation
step along the cone to accumulate light while multiplying the result with alpha
stop if alpha is zero
For refraction, you need to refract the cone (the BRDF is replaced with a BTDF)
You should consider contacting the author for a better answer