an inositol-1-phospho-N-(2-hydroxy-very-long-chain fatty acyl)-4R-hydroxysphingoid base + GDP-alpha-D-mannose = an alpha-D-mannosyl-(1<->6)-1D-myo-inositol-1-phospho-N-(2-hydroxy-very-long-chain fatty acyl)-(R)-4-hydroxysphingoid base + GDP + H(+) — reaction