Involved in the latter stages of the biosynthesis of the alpha-(1->6) mannan core of lipomannan (LM). Catalyzes the addition of alpha-(1->6)-mannose residue. Cells lacking this gene show a complete loss of LM biosynthesis. Mutant results in the formation of a truncated LM with a reduction of alpha-(1->6) and alpha-(1->2) glycosidic linkages. Belongs to the MptA/B family. Extended N-terminus.