To the best of my knowledge, there is no way to load external shader scripts in Godot.
You may be able to make a Shader object (documentation) and change the code property to a string you load from a file using a File object (documentation, saving games page from documentation). I'm not sure how easy it would be, nor if it will work, but it is something to try if you want to load shaders from external text files.
Also, to the best of my knowledge shaders in Blender are written using GLSL and/or OSL. While Godot's shader language is similar to GLSL (documentation, migrating guide ), there are some differences that probably will make it where you cannot use the exact same shader in both Blender and Godot. That said, I'm not terribly experienced with shaders, so maybe it will work fine.
If I were you I'd make a simple test in Godot with your shader code, and then if you want/need, maybe look into making a system to load shaders from external files using the File class or something similar.
