Editing skysphere material?

Would it be straight forward to edit the skysphere material to include objects such as buildings or mountains?

How do you reference where things are in the material and their location in the world once rendered?

How do you create a cubemap from 6 separate images without the use of photoshop, is there a free alternative ie gimp?

I’ve tried 360 toolkit and my sky in game

looks like this:

I’m trying to use the following but dont have photoshop…ortingCubemaps

The skysphere >> Sky material >> “Material Instance Dynamic” can’t be disabled either?