One way to do it is with custom node as Ryan explains in this video: Custom Material Node: How to use and create Metaballs | Live Training | Unreal Engine - YouTube where you nest two custom nodes.
Currently this trick only works for DirectX shader compiling path, while OpenGL and Vulcan is still not working and it is reported.
Another way is to create a global shader plugin in C++ where you can expose that function, which is the best approach since it is portable to other platforms. You can follow this thread to guide you: https://rcaloca.blogspot.com.br/2017…rs-to-ue4.html
Also, could you post the result of your test with the change, so others can use the info as reference?