Catalysis of the reaction: a 1D-myo-inositol-1-phospho-N--(R)-4-hydroxysphingoid base + GDP-alpha-D-mannose = an alpha-D-mannosyl-(1,6)-1D-myo-inositol-1-phospho-N--(R)-4-hydroxysphingoid base + GDP + H+.
inositol phosphorylceramide mannosyltransferase activity