This should be really easy to do. Just use the "wireframe" overlay's alpha channel for blending.

ex: (HLSL pixel shader)

float4 inColor = tex2D(mapSkin1, In.tex);
float4 inGrid = tex2D(mapSkin2, In.Tex * oScale);
float4 outColor;
outColor.rgb = lerp(inColor.rgb, inGrid.rgb, inGrid.a);
outColor.a = inColor.a;

Of course some of the variable names will need to be renamed, but this is the concept for it...