Yes, Posca pens can be used with stencils or stamps. Simply shake the pen well before use, then apply the paint through the stencil onto your surface in a dabbing motion. Allow it to dry fully before removing the stencil. When stamping, load the stamp with paint, then press firmly onto your surface.
The acrylic ink will give crisp edges. Just be sure to clean the stencil or stamp right after using to prevent drying.