A generalised approach for modelling the morphology of molecular crystals in the presence of "tailor-made" additives of both the disrupter and blocker types is presented and demonstrated by a consideration of the effects of a series of eight "tailor-made" additives on the morphology of the long chain normal alkane docosane (C22H46) The additives considered include n-alkane homologues of different chain lengths (both longer and shorter than the host) along with methyl- substituted n-alkanes. Considering other n-alkane homologues as additives demonstrates the ability of these materials to incorporate easily into docosane crystals although the habit-modifying properties are not great due to the similarity with the host sytem and the anisotropic nature of the bonding. The predicted crystals are thinner along the c-direction compared to the pure system, although they increase in thickness with chain length from n=20-24. It is shown that addition of methyl groups to n-alkanes can have a significant effect on their morphology forming more equant crystals as the number of methyl groups is increased. The asymmetry of the additives greatly affects the relative growth rates of pairs of faces (hkl) and (-h-k-l) leading to a polar morphology.