EDIT: this contribution has received downvotes, and I guess I was not clear enough. Even if I do praise probity, academia is not homogeneous, neither do so-called scientific fields. Authorship, work, citation and evaluation habits differ a lot. So I will describe three practices I have witnessed. Completed by the conference Evolution of Scientific Publications: the Point of View of Scientists (in French, Colloque de l’Académie des sciences, 14-15 mai 2007), which I attended.
First, maths. Traditionally, many consider the contributions are equal as soon as people co-author. And the authors are put in alphabetic list. Yet I know a person who gained a professor title from a developing country, and was offered a certain amount of money to add local co-authors to a paper. Who did not have the level to contribute. Which the person accepted.
Second, in my community, some consider the student ought to be first author, even though the student did not do most of the work. The student should find a position, after all. The advisor is often put at the end (even if she/he did nothing). I was once strongly suggested (by a boss) to add somebody else, because "he was important". I was young, and did accept. I do regret that now. After all, rules in our field are not so strict as mathematicians...
Third, in medicine (a close friend's experience). Papers can have much more authors. My friend told me (exaggeration) that the first author writes, the last proof-read, the others are... there, with varying good-will and contribution. Indeed, in the above mentioned conference, I have seen in a row a mathematician claiming that authors are of equal importance, and a biomedical pundit asserting that: the first place and the second are important, and the last and the penultimate too. Indeed, I know at least a ranking in France that rewards services if a member is a co-author of a publication with different weights: w1 for the first and last, w2 for the second and penultimate, w3 for the third, zero for the others. I let you dream about the fights to change positions from 4th to 3rd... And I know an author who often add (his/her) spouse to the co-author list, even if the spouse is not knowledgeable in the field. And the other authors accept that.
So there are habits, conventions, pressures of different sorts. And the borderline is easily crossed, and fluctuates across fields.
If you are strong enough to make your own rules, do want you want, at the risks of hitting local habits.
So you "can" add a co-author at this stage. A "revise and resubmit" is a new paper. In some fields, this is common practice, but might be borderline inappropriate if you just add a "name" to help the paper go through the next review.
To improve your paper impact, and strengthen your ability to cross disciplinary borders, I would suggest you to re-elaborate the novel paper (collaboration = work with) with your putative co-author, instead of performing a mere scaffolding, probably less painful, but likely, in the end, to be less beneficial.
