Who owns the rights to the Duke3D expansions? They weren't 3D Realms games, so I doubt we'll get them on here.
It's not impossible. According to the Duke Nukem wiki, Wizardworks was bought and disassembled by Atari, so it's likely they have the rights to these expansions. And GOG are currently selling Jagged Alliance 2: Wildfire, so it wouldn't be the first time they sold an expansion that was made and published by different companies.
I think the main obstacle to these expansions being added to the library is whether the demand for the expansions is big enough to convince GOG and Atari it will be a profitable venture.