Choose a blank level, and use the "merge file" function to bring in the solid of "as cast" into the "as machined" file. I immediately save my combined files as "workfile", to avoid writing to a file currently in use. If not oriented correctly, finding 2 or 3 features on either model, setting WCS to a common plane, via face selection, and creating, say, points that can be used to further "tune" the WCS origin of the stock solid to match the machined version WCS. Use "save some" and resave the merged solid under an .STL filename with the WCS selected. Load the original "as machined" file, and in Toolpath manager, select "from file" in machine group stock definition, point to the file, and I think you will be off and running. There is without doubt a much better way to do this. Hoping the above will bring forth some more elegant solutions, from those with a higher skill level.