TL; DR: Yes, and no.
No, you really can't strengthen the connections of the ribs to the sternum and/or spine. The ribs (1-7 from the top down) connect with cartilage to the other bones of the chest/spine. In the cases of ribs 8-12, they either connect into other ribs or are floating, i.e. they only connect to the spine. This part can't be strengthened, and unfortunately, that's usually the part that separates. Best advice, don't let your opponent kneel on your chest. (I know, I know, I roll regularly too.)
However, what you can strengthen is the muscles that run in between each of the ribs, that assist with structure stability and breathing. These are called the intercostal muscles. You can't work them directly, but there are a couple of exercises that work them in stabilization.
While primarily a lat exercise, straight arm pullover/pulldowns will also work the intercostals, especially if you take care to tense your ribcage as you do the exercise. I like dumbbell pullovers (laying flat on a bench), as I think it gives you a broader range of motion and stretch than a straight arm pulldown on a lat machine.
Any type of dumbbell fly will also work the intercostals. Basically almost any exercise where your arms are straight and move away/towards the chest will do, so incline/decline/flat bench flyes will all work the rib muscles as well.