It'd be nice if you had a tutorial or at least example of how to add cutom gui's to mod config file. It's been two days straight since I was trying to figure out how to add gui's from CustomNPC's and FancyMenu.

Hi V4NK0V, sorry for the late response:
To add custom screens to the config you need the internal name of the screen to add. If you don't know that you can try this:

1. Open the settings and change "Enable Debug Mode" to true. This should make an overlay appear on the top left of the screen whenever you have a screen open.
2. Navigate to the menu you want to add and note down the topmost text (e.g. appeng.client.gui.implementations.IOPortScreen). Alternatively copy it from the Minecraft log. That is the internal name of the screen.
3. Add an entry to "Custom Mod class names" and paste the internal name.

Now the screen should be paused. If it works just disable "Enable Debug Mode" again.