When to add diced vegetables when cooking rice
It depends a lot on the vegetable. For frozen peas, I add them just when the cycle finishes; fold them in and close the pot, and 3 minutes later they are steamed to perfection. For a harder vegetable, like a raw carrot, or one where you want more breakdown, like onion, add it at the beginning. 

You don't really want to open the rice cooker a bunch of times during the cycle as it loses steam and may affect your results, but if you have vegetables that you feel need something in between, I wouldn't hesitate to open it once, midway through.