The main goal of this paper is to extend an existing biomass characterization method, which, on the basis of the elemental carbon, hydrogen, and oxygen composition, derives the biochemical composition, in terms of cellulose, hemicellulose, and lignin. Because of their elemental composition, several biomasses cannot be characterized by a feasible mixture of these reference components. This limitation is removed by accounting for the presence of a couple of new lumped species, representing hydrophobic and hydrophilic extractives. This extended characterization method allows one to enlarge its range of validity significantly, thus covering most of the biomass samples. The accuracy of the extended method is validated by comparing experimental data on structural or biochemical composition with model predictions. The multistep kinetic scheme of biomass pyrolysis is then extended to the new reference species, on the basis of comparisons with TGA experiments on triglyceride and tannin pyrolysis. The new pyrolysis model is proved to be fully consistent with the previous one, already widely validated under different conditions. Several comparisons of pyrolysis of biomass samples, outside the applicability range of the original method, validate the proposed extension. Moreover, the results show that this predictive model is not very sensitive to the different degrees of freedom of the new biomass characterization approach.