If I purchase additional modules for my B1500A do I also have to pay to have Keysight install them?

Yes. The cost of installation is not included in the price of module(s) purchased as stand-alone item(s). So, please contact your local Keysight Field Engineer for the quote. And when you receive the stand-alone module(s), please send both the new module(s) and your B1500A to your local Keysight Service Center.