diagram of attenuation
It is not really possible to "convert" a band diagram into a transmission/reflection diagram. The band diagram assumed your photonic crystal was infinitely periodic. A transmission/reflection diagram typically assumes infinitely periodic in x and y, but z is finite with some number of unit cells stacked to form a slab.
With that said, you can "estimate" the transmission spectrum through the density of states. Think of summing all the bands for a given frequency and making the number of bands the measure of transmission. It is crude, but surprisingly accurate. The reflection is then R = 1 - T.
-Tip