
Adding such things can be done but, depending upon the type of metal roof you have and the location of the addition, it could get very involved. You should have a product warranty from the supplier and an installation warranty from the contractor. I can't say for sure as I have no idea what the wording of those warranties is but I would not be surprised if the product warranty would remain but the installation warranty would be voided unless the same contractor does the additional work.
If I have a roof installed and a few years later (still under warranty) and then I want to put in a skylight or chimney, is that possible? Does it change/void the warranty? Would I need to use the same roofing contractor?