I may be the only person in the universe who's played The Temple Of Elemental Evil (the computer game, not the tabletop game). It's a genuinely turn-based D&D game, in contrast to BioWare's real-time D&D games.

TTOEE is terribly buggy and has severe user interface stupidness that makes it irritating to play. But, I find the turn-based combat so enjoyable that I keep playing it despite the major flaws in the rest of the program.

Neverwinter Nights and TTOEE follow nearly the same ruleset. You can, and indeed people have, reimplement TTOEE as a Neverwinter Nights module. It's certainly possible to take a turn-based game and transform it into a real-time game.

The most obvious mechanism is to remove the 'round'. You have turns, but have each participant take their turns without having to wait for anyone else. The usual rules about how long actions take can be used as normal: in D&D you know how long each action takes, in a custom system you need to be sure you know the same.

It should be obvious that it's easier to go from a turn-based system to a real-time system than to go the other way. If you start with a real-time system, then you can easily run into problems where things need to be happening at the same time.