Catalyzes the alpha-1,2 addition of a mannose residue from polyprenol-phosphate-mannose (PPM) to a monoacyl phosphatidylinositol pentamannoside (AcPIM5) to generate a monoacyl phosphatidylinositol hexamannoside (AcPIM6). Phospholipid metabolism; phosphatidylinositol metabolism. Belongs to the glycosyltransferase 87 family.