How do I time-stretch a signal from the frequency domain?
Are you talking about taking an audio recording where someone was speaking too fast, and you want to slow down the speech without changing the pitch? If so, then your original formulation of the problem is incorrect; you can’t do what you want by taking one long FFT of the entire recording and then processIng it.  The usual way this is solved is by using the phase vocoder, where you break the signal into overlapping blocks and transform each block.  This is a deep subject with lots of tricks to avoid artifacts.