It's already baking now! Can I just leave it in longer?
It's hard to say exactly since you didn't bother to post the recipe or provide a link.
That said, most casseroles are by nature a bit imprecise in regard to cooking time, so you should be able to leave it in a little longer. If you are concerned about the top getting too brown, cover with foil.
if they have sauce around them, i don't think baking them longer will work. If they don't have sauce on them, add 1 cup water or chicken stock to them, cover with foil and crank up heat to 450 degrees, which will cause them to cook in steam. Check them after 20 minutes.
Microwave them in 1/2 cup water, covered tightly with plastic wrap, for 1-2 mins. They'll steam and be cooked enough for your casserole. I use the veggie cooking bags....they work really well.
That doesn't help once the beans are in the casserole and already bake as the original poster stated.
Hope leaving it in longer work for you because I'm in the same boat now!
For those looking for a follow-up I did the same thing and putting it in the oven just longer worked