Can you add extra capacity to a Modular UPS without a system shutdown?

Yes, and this is a key feature of a modular UPS system. The UPS modules are hot-swappable modules. Modules can be removed for service and maintenance and extra modules added without the need to power down the UPS system. The UPS continues to protect the load during module removal or insertion into the UPS frame. When a module is inserted, the system goes through a number of self-tests before the module is accepted electrically into the frame and brought online.