Epoxy can be applied over fully cured, unwaxed shellac only. Waxed shellac will prevent the epoxy from being able to bond to the surface. It may be helpful to scuff the surface of the shellac very very lightly with a Scotch-Brite pad or 320-grit sandpaper, being extra careful not to remove the finish. The surface also must be clean and the shellac completely cured.
To ensure that this achieves the desired finish and end result, it is strongly recommended to always perform a test sample prior to attempting this on the final project.