You can create components within the same project using sub-components.
To create a subcomponent simply highlight your desired component in Haiku and right click on it to "Create Component".
Once you've created your component you will see it available as an editable and reusable component in a new tab on top of your Stage and in the menu under Components.
To use your components, simply drag them onto the Stage from the left hand menu the same way you would with an imported element.
Coming soon: the ability to share components between projects, and pull in components from other users; all from within the app.