How can I add meat into a completed tomato sauce? Yesterday I was cooking for someone and was planning on making a bolognese sauce, but when it came to start cooking (ie. after doing my mise en place), she asked if I could leave the meat out of it, so instead I ended up with a meat-less tomato sauce with various veggies in it (onion, garlic, green pepper and carrot; I'm not sure if there's a "proper" name for this sort of sauce) -- essentially, my bolognese recipe minus the meat. I mixed my pasta into the sauce a couple minutes before serving.

Today I have plenty of leftovers (that have been sitting in the fridge for about 24 hours), and would like to add my meat (ground beef) into it. I realize that instead I could do something like make meatballs, but I'm not looking to do something like that.

Would just browning the meat now and mixing it in work well? Is there something I should add to my meat to better incorporate it into my sauce? Also, I'm not sure if putting the sauce back on heat will be the greatest idea because the pasta unfortunately got cooked more than I wanted (to the point where it's okay as-is, but further cooking will probably push it to overdone).
As the others have mentioned, it won't quite be the same ... however, it can be done, it just won't quite have the same flavor as there isn't a chance for the meat juices to transfer into the sauce, and some issues regarding mixing it back in with the cold pasta.

If I were going to try it, I'd brown the meat, remove some of the fat if it's a really fatty grind, then add some extra sauce (or even other liquid ... maybe dairy, for a classic bolognese) to let it simmer for a bit ... then toss the pasta in with it to reheat.

The extra sauce is the key, otherwise, the meat isn't going to blend in with sauce on the pasta,  If you reheat the pasta in the fresh sauce, it should hopefully mix together more completely.

... but if I were to do it, and wanted to add a meat to an already cooked pasta, I'd probably not choose ground beef as a first choice; I'd probably cook up some sausages, cut it down to an appropriate size for the pasta you're dealing with, and mix that in, possibly adding some extra liquid when reheating the pasta.