Well, NetHack has been ported to everything from the Amiga to OS/2, so I don't doubt it's possible. NetHack is a sleek 3.6 MB, so space shouldn't be a problem, as the Nspire has more than 4 times that much storage. The game is written in C, so only one question remains: How good is Ndless's C compiler?
If NetHack was ported, I would go out and buy a TI-Nspire tomorrow. It's a great game, and if I was able to play it during math class I might finally beat it.