Can I add milk to tea while it is steeping?
The big issue is temperature. There are various guidelines for setting temperatures for various kinds of tea, but the most common advice for traditional black tea is to use boiling water. If you add cold, or even warm, milk to the tea it's certainly going to alter the rate at which various flavor components (caffeine, tannins, polyphenols, catchins, etc) dissolve. 

It is primary a matter of solution rather than actual chemical changes. Tea is toasted before you get it; black tea is the most cooked. You aren't engaging in many more changes during the steeping. It's just a matter of what dissolves out when and at what temperature. Compare cold-brew coffee, which leaves behind many of the more burnt/bitter flavors of a roasted bean, as well as less acidity and caffeine. Some people like it; others don't. There are a great many chemicals involved in both, all with different rates of solution, and some people put great care into getting the temperature and brewing time to their taste.

You can also heat the tea in the milk. That's how traditional masala chai is made. Generally, you won't want to boil it for long, since that will cook the proteins in the milk, rendering it clumpy. It's often given a long simmer at a temperature somewhat below boiling.