Increasing mesh resolution in specific areas of a model will normally introduce triangles at the low res/ high res interface, but if you squeeze a quad poly into your poly loop diagonally you can add two more columns of polygons, without adding any triangles.
Sadly I don`t know any way to do this in Z-Brush. It possibly can be done, using the point welding option in the object import panel creatively, but really this sort of mesh editing is best done in a traditional 3D package.
Hope this helps some people a bit.
Cheers,
R