Is it appropriate to add a co-author at the stage revise & resubmit? Context
I've submitted a paper - it was sort of a side-product of my main project - to a high ranked journal from an adjacent field. Recently I got a revise &amp; resubmit that was close to a reject, but the editor sounded really interested in the paper being multidisciplinary. I'm suggested to contextualize my research stronger within the journal's field. I feel like getting one of my experienced in that particular field colleagues involved in the revision in order to get the paper accepted.

The question
If a person helps you a lot during the revision, are you allowed to include him as co-author?
Absolutely you can add a coauthor in revision, if there is good reason for doing so: in fact, I have recently done so myself on a paper where revision involved some new experiments with the aid of a colleague not previously involved.

Most journals that I know consider authorship changes routine and even explicitly support such changes by asking you to review and, if necessary, modify author lists as part of submitting a revision through their system.