Is it possible for a modified hang glider to land safely coming from orbit?
That's pretty much how space shuttles and other orbiting craft work.

They are dropped from orbit, they do calculations to enter the atmosphere at a velocity and attitude that doesn't burn up the aircraft, then they glide to their landing airport.

The entire journey is controlled, but unpowered flight. The flight surfaces are there to allow the pilots the ability to reduce the velocity and attitude of descent in order to get controlled descent into the atmosphere.

So a glider can also land; assuming of course it had some way of slowing down and is sufficiently hardened and shielded against the atmospheric effects. I am not too familiar with gliders but from what I know they don't have any spoilers or other such controls.