I don't know if inside or outside is better for her. I've never dealt with or read up on pneumonia in horses. What did the vet say about it?
7Ponies had a couple of great ideas. I'm assuming your horse is standing and not lying down. If you are bound and determined to get her inside, there are ways to force the issue -- driving behind her is one way, but be very aware of where contact is made between the vehicle and the horse -- you will likely need a very soft padded attachment at a different height than the bumper.
Does she load in a trailer easily and could you get a trailer to her? Can you get her to move sideways or will she just allow herself to fall over? Sometimes you just need to get the movement started, even in the wrong direction. Have you tried her favorite treats, if OK'd by the vet?
If she stays out all night, make sure she has water close to her. You maybe could rig a wind shelter near her using tarps between trees or something.