Adding a UV offset to tex2dproj.

I'm trying to create a blur effect that will only be applied to whatever is behind a specific object. For that purpose, i have created a shader that uses GrabPass to get the rest of the scene behind it.

 tex2Dproj(_GrabTexture, UNITY_PROJ_COORD(IN.proj));

IN.proj was initialized with float4 proj : TEXCOORD;

The issue is that i don't exactly know how to add an offset to the second argument in order to sample neighbouring pixels (something like tex2D (_GrabTexture, IN.uv_GrabTexture+half2(_GrabTexture_TexelSize*-1, _GrabTexture_TexelSize*-1) )