You can change the individual layers textures as you can see in this picture on the left https://cdn1.epicgames.com/ue/item/TropicalIslands_Screenshot_06-1920x1080-1d96bd496355350ba146abb87af59f9d.png . But these textures are specially crafted to work with the system so just taking some textures and putting them into texture slots without editing will not result in a perfect final look. The system doesn’t utilize the standard setup of diffuse, metallic, roughness and normal map. It is based on a desaturated defuse map edited with a high path filter that is colored by a vector parameter and the metallic and roughness are computed in the material. The system implementation for the dynamic landscape material is rather different to your normal material setups.
So the answer is yes but probably not in the way that you currently think you would use them. However you have full control over everything that happens in the landscape material via the instant materials and if you want to you can edit the master material itself to do whatever you want inside of it.
But usually there is no need to actually build your own textures in order to use a system since there are already several texture variations included that you can just adjust to your needs.