You can, but the calzones are baked only for 15 minutes, which means that the vegetables might remain uncooked. In that case, then use vegetables that don't really require much cooking or that can be eaten on the raw side, such as spinach, red or sweet onion, cherry tomatoes, red pepper, canned artichokes, etc. In that case, make sure to make an opening over the calzones so that the extra moisture can escape.