Somebody correct me if I'm wrong, but I believe the only thing that's needed for a planet to be a terraforming candidate in Elite is to be within the star's habitable zone. (Or stars', in case of multiple stars that are close enough.) At times, you can get some pretty hellish worlds that are still candidates. But these atmosphere-less planets do have a metallic core, so I'd assume they also have a magnetic field. With that in place to protect the planet's atmosphere from being "blown off" by its star, I think it would be easier to terraform a planet without an atmosphere than it would be to terraform one with, say, an atmosphere of 100% sulphur dioxide with a surface pressure of 150 atm.
Then again, Mars has been terraformed centuries ago in Elite, so I'd expect that by the 34th century terraforming technologies are very advanced. And if they did involve the earlier quoted suggestion of bombarding the planet with some icy bodies, that would be pretty cool. After all, why spend credits on landing all that mass gently when you could just deorbit them and let them crash?
Hm, a thought just occurred to me. According to lore, humans probably first ticked off Thargoids by starting to terraform some ammonia worlds of them. (Veliaze.) I wonder if they did that with some orbital bombardment too.