Hi guys. I recently read about some volume rendering papers and noticed that there are 3 types of behaviour when light interacts with volumes: SCATTER, ABSORPTION and EMISSION. But the default volume shader core and pyro shader core (in Mantra) doesn't support Absorption. Is there any reason for this or how do we get around that?

You can do it, you just have to refactor the volume material to be based on those parameters. As it is, it uses density and diffuse albedo.
if my memory is correct the relationship is:

scatter + absorption = density
scatter / (scatter + absorption) = diffuse

you should be able to change the signature parameter to vector. to get absorption