I accidentally said "no" to the reporter in the Citadel, can I change my mind?

She's Diana Allers.

I tried this out (telling her no) and after she left I was not able to find her anywhere in the Citadel, so it appears that it's not possible to recruit her once you decline her offer (I doubt I'll find her later in the game).

I'd suggest you try to load an earlier save, as having her aboard boosts the other war assets, even though she's only worth 5 (on the other hand, some of her dialogue is annoying, so it's up to you, it's not game breaking one way or the other)

No. She is a missable war asset but, to be fair, she is only worth 5 points and a romantic interest.