/Bob wrote:
We've discovered that part of our 30+ year old house has no boxes
behind the electrical outlets. The switches and outlets are just held
in position by the stiffness of the wires. Is there a straightforward
way to add boxes "after the fact" without tearing out wallboard?

Sure. Home Depot has a selection of blue boxes that have ears that turn
out behind the wall when you tighten the screws.