It's unfortunately not possible to manually add a stage to a project.
When a project has been created in the Project Manager, all stages are copied over from the Project template. It's not possible to manually add new stages to the project after it has been created.
Expand project
SyncForce supports the functionality of Expanding your project. This means that a project can start of with just 1 (or more) stage(s) like 'Feasibility'. When this stage is done, the project can be 'Expanded' to be a New Product Development project or an Existing Product Development project for example. Whether a project can be expanded is defined in the Project template configuration.