It's definitely possible, but with an exception.
While object shaders work in FPS Creator without any source modification, fullscreen shaders need a mod which supports the loading of custom fullscreen shaders, like Project Blue.
If you purchased Model Pack 22 you'll find a template and video for your own shaders using DarkShader, located in "DarkShader Template" inside your main FPSC Directory.
Quote: "but for this you need a render for deferred shading system"
What are you talking about? You don't need any kind of deferred shading for that, DarkShader already comes with special commands to load the fullscreen-shaders...they can be loaded with native DBP Commands, anyway.
-Doomster