This forum lists addons available to Construct: plugins (objects), behaviors (e.g. movements), and pixel shader effects.
Adding a plugin or behavior
Plugins and behaviors are stored in two .csx files: one for the editor, and one for runtime. First close all instances of Construct, then copy the files to the Construct install directory under the Plugins folder and Plugins\Runtime folder respectively. When you restart Construct the new plugin or behavior should be available.
Adding an effect
The process is the same as above, but you will be copying a .fx file to the Effects directory.