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