Adding co-author for a revised manuscript
A revision to a manuscript is a chance to correct any issues raised in the original review. If you and your co-authors feel that adding a co-author would make the manuscript better, then it is of course acceptable to do so, absent explicit instructions to the contrary from the journal (although I am unaware of any journals with such rules).

However, you will also need to clearly explain in your cover letter to the editor why you added the co-author, and exactly what her contributions to the paper were.