If I unuderstnd your question (combined absorption and scattering out of known complex refractive index, that is losses) you only neeed to use the complex part of the refractive index as the extinction coefficient. Beware of having the right (complex) index at your frequency of operation.

Hello Dr.Royo, Thanks for your answer. 

Through k=2(pi)n/(Lambda). You'll get an exponential in z. If I get your problem right, not totally sure...Try

http://en.wikipedia.org/wiki/Refractive_index

at the complex refractive index entry.

Thank you very much Dr.Royo