Catalysis of the addition of mannose to dolichyl-pyrophosphate Man7GlcNAc2 (also written as Man7GlcNAc2-PP-Dol) in alpha-(1->6) linkage, producing Man8GlcNAc2-PP-Dol.
dolichyl-P-mannose:Man(7)GlcNAc(2)-PP-dolichyl mannosyltransferase activity