Historically you have been able to add cruisenext vouchers to existing bookings, and as far as I am aware this is still the case. We last did it about 6 months ago.
However, you should be aware that the T&Cs state that they can only be used on new bookings, and this wording was changed recently to make that clearer. At the time some people took this to mean that they would start enforcing it, but this hasn’t happened yet.
The point is that, whilst it can currently be done, this doesn’t mean that you will still be able to do next month.
I have always applied the vouchers online rather than call, as I recall that some people have said that they couldn’t get done by phoning as the rep wouldn’t apply them (I assume because they were following the rules). I believe that hanging up and trying another rep would often work.
If NCL ever fix their IT then this loophole may close. 🙂