Make Imported 3D Objects Hollow in the Middle

Hi friends! I have a really nice building asset I imported from Unity Asset Store.

However, the building is almost cubed shape and there’s a mesh collider. I was hoping to maintain the looks of the building while cutting out one chunk of the building as an entry point, so my character can enter the room.

Does anyone know how I can do that? Much appreciation for any help!

I think the only way is, to import the model into blender or any other 3D software and edit it there…