Combine Colormap + Splat

Hi, we've been working for the past year with CryEngine and are curently moving to Unity. When working with terrains, we've encountered a difficulty, which didn't occur in Unity. We need to apply detail bumpmaps to the terrain, but leaving the Satellite image we used as colormap unchanged. How can we achieve this, considering the terrain paint tool asks for a diffuse terrain texture, and won't accept only a normalmap, and doesn't have an option to set opacity for diffuse texture to 0 when painting? Is there any custom tool in the Asset store than cana chieve this? And one more question: is it possible to apply a splat so it can be seen when close, and replaced by the original colormap when seen from far away? Thank you!!