Would it just be, like an add-on construct? Could it look realistic or would it be green?
I’m pretty sure I’ve seen Lanterns make suits or mech type constructs that cover their whole body. I think that could be used to work like shapeshifting. Or if there was some sort of gel or ooze based alien race they could probably use their ring to restructure their body.
Yes they can and they have many times. It normally looks very convincing.
So I believe Green Lanterns work by light construction. Yes they are able to fool people by their appearances by altering light rays to look like something else but not necessarily shape-shift like Martian Manhunter or plastic man as they bend their actual body shape.
No, there have been cases of them making themselves enormous.