Here's a good image of the items you'd need for a 2 liter carbonation setup.
I do that sometimes, but there's a few tricks to it. Because you lose carbonation when you initially open a bottle. (Like opening a 2 liter, closing it and leaving it, just tends to be flatter the next day)
You either have to overpressurize, so that the carbonation equalizes back to the desired pressure, or chill it down close to freezing so that you lose as little carbonation as possible.
Of course, if your disconnect is leaking, then it'd be better. Mine's not leaking at the moment (knock on wood) so I find leaving the disconnect on to be better than trying to replace it with a cap. I've had 2 x 2liters travel before, one with the carbonator cap and one with the regular cap replaced (at cooler than room temp, but not close to freezing). The carbonator cap had more pressure in it at the end of the trip.