FBX is the file we use to export from Max to Unity, but you can use any native mesh in Unity, once Forest is imported.
A simple procedure, assuming a single mesh by Forest object, would be:
- Drag the FP asset to the Unity scene.
- In the Hierarchy tree, you can see the Forest asset as 'fp1'. Expand it.
- For each Forest item, there is a 'fp1(Clone)' element. These are the Forest instances. Select all of them.
- In the 'Inspector' window, check the 'Mesh Filter' section.
- Click the circle icon (just to the right of the Mesh name), and pick the Unity mesh you want to use. All items will be updated with the new mesh.
Probably there are other ways to do it, but unfortunately our Unity knowledge is limited.