Is it appropriate to add a co-author at the stage revise & resubmit?
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.