Polyurethane IS varnish; it just uses synthetic resins and/or oils rather than natural ones. Once varnish is cured, additional layers added to it will only form a mechanical bond. There is no chemical interaction and no compatibility problems. What you will need to do is sand the surface to smooth out rough spots and provide some "tooth" for the next layer to adhere to. It doesn't matter whether the current finish is polyurethane or not, or which type of finish you apply over it.