There is a lot of emphasis on having adequate return air and possible remedies. So let me add to the questions.
With regard to grills, if the duct remains the same, what is the effect of adding a grill with greater free area? Lower velocity through the return run, or is that seen only at the grill? What about a second grill (different location) added to the existing run?
When adding a new return run, must it be connected at or relatively near the end of the return trunk or at the plenum? Where questions have been asked about the feasibility of adding a supply branch to an existing branch, the response has always been not to do it as it might negatively affect the original supply. So would doing so with a new return be a no-no?
Adding to the existing return could effect it negatively. Depends if the existing return grilles are sized right or not. If they are undersized. It won't hurt.
often best to run to the plenum. But not always practical.
Increasing return grille free area, is always a plus. It decreases the return static the blower must work against. And often increases the supply air flow(VS blowers can be an exception to increased air flow).
By checking the current static pressure in teh return duct. You can tell if adding to it will hurt or hinder.