OpenSCAD linear_extrude from multiple path svg import
Lame solution: creating stepped pyramid with offset. I realized that scale will not worked for motives with holes inside. Offset seems then more appropriate than scale for my application (creation of stamps)
It takes ages to render, but it could be enough for simple patterns. Any better solution are still welcome...
module buildPyramidalExtrude(height,maxOffset,nSlices){
    heightIncrement = height/nSlices;
    offsetIncrement = maxOffset/(nSlices-1);
    for(i=[1:nSlices])
        linear_extrude(height=i*heightIncrement)
              offset(r = maxOffset-(i-1)*offsetIncrement)
                children();
}

buildPyramidalExtrude(4,2.5,20)
   import(file = &quot;Farm/cow.svg&quot;, center = true);