PrestaShop Cloud is a SaaS service provided by PrestaShop Team, you can host there your shop with fast and easy in use one-click installation. As you probably noticed - there is no upload form for modules downloaded from addons store, or from other marketplaces.
Anyway, you can upload there non-default modules, the one thing: modules must be trusted by official addons marketplace. If you want to upload trusted module, please follow this guide: upload modules to prestashop cloud service
