This question already has an answer here:
Can the Android Layout folder contain subfolders?
The folders in
/res/ have a predefined structure, and you can't add subfolders.
However, seeing as this is the
raw folder, you may want to look into using the
assets folder, in which sub folders are supported.
Keep in mind thought that by using
assets you won't be able to use
R.*.* references anymore. You will have to go through the AssetManager.