When I got the choice to take back Imoen, my party has already got 6 members. So I kicked out Nalia.

Is there any way to bring her back to the surface again?

Ahh the classic dilema of who to leave in Spellhold...sorry, if you choose to dump Nalia then Nalia remains and you won't get her back in SoA. Likewise, keeping Nalia and telling Imoen she is a big girl and can find her own way out is another possiblity...one that allows you to keep Nalia and pick up Imoen later...

You must do what you feel is right of course...

If you've already left the maze, then I'm afraid there's no getting her back -- otherwise, take a look at these links (from the archives) to see how you can get all party members out safely (spoilers at own risk):

"stranding someone in spellhold/asylum" http://www.sorcerers.net/ubb/Forum2/HTML/001444.html

"Dumping Imoen" http://www.sorcerers.net/ubb/Forum2/HTML/002288.html

If you *have* already left, but desperately need Nalia for her sparkling conversation, you'll have to resort to a previously saved game. Good luck!

Is there any 'cheating' way to make NPCs back to the surface from Asylum? eg, using Shadow Keeper?

Nope... ShadowKeeper can't help you with this one. Only some *more serious* editing or cheating can - and that's pretty much complicated. Very complicated. (Numbers, Numbers, Numbers...)

You could cheat by getting back to Athkatla, leaving someone in the Copper Coronet, using the console to jump back to Bodhi's maze, picking up the straggler, using the console to jump back to the Copper Coronet again, and then releasing the moron NPC who couldn't follow you out of the dungeon in the first place.

But if you're just going to be cheating anyway, isn't it easier to pretend you brought the seventh person home with you and then dropped them off at a pub somewhere?