I have never done exactly what you describe, but can see no reason why it wouldn't work. I understand you to be saying that you don't want to carve the gun stock itself, but only apply some carving to an existing one. The hardest part will be to align and clamp down the existing object so the CNC and Aspire know where to start. There is a nice capability of dropping 2D tool path onto a 3D surface, and if your carving is basic 2D/2.5D text and patterns then this is what I'd try. You won't want the CNC trying to carve the existing, surrounding surface along with the carving you've merged onto it.