From TDN

Customizing Constructor

Prefabs

Prefab objects are essentially parts of Constructor scenes that have been saved out for use in multiple other maps. Commonly repeated objects such as columns, crates, door frames, and windows make excellent candidates for Prefabs.

Prefabs can be found in the Tools panel's Prefab tab.

Custom Brush Creation Tools

All of Constructor's brush creation tools are script driven. You can build your own tools to create custom shapes, if you are so inclined. Any of the default brush creation tools, along with the example User Tools are excellent examples to start from.