Making Prefabs with new origin from imported model?

Hello, I imported an entire scene built in maya into Unity. Now I want to optimize the scene so I am trying to create prefabs of items that repeat through out the scene. for example, i have some trees that repeat through the scene. I dragged the tree into the project folder to create an asset, but the model seems to have an origin at the center of my imported scene model, not the center of the tree. The prefab is also rotated. If I drag the prefab into the scene, I can't see it in the window because origin is off.

I realized that the prefab wasn't being created correctly when I tried to replace game object instance from my original scene model with the prefab i created.

Is there a way to correctly make a prefab from an imported game object with a new origin and maintain its rotation value? Thanks.

Also noticed that some of the prefabs don't have object previews (because the are off origin?) so I tried the reimporting them but it didn't make a difference. some of teh prefabs have previews, but all of them are off origin and rotated.