He would need to do A manual j and manual d for the whole house including addition... Not just adding duct and some dampers.. The whole dust system will more than likely need to be redesigned or you may need to upgrade system size. This is what manual j will tell you. Then you will have a option to use manual dampers and balance system properly or you do a complete zoning system... You can't do both.. But as long as its designed properly then system should thrive..