Create a child domain

I have a root domain as abcd.com
I have four domain controllers – two a site A, one at site B and one at site C.

I want to create a child domain xyz.abcd.com and promote DC at site B and site C for this child domain.
1. Can I use existing DC at site B and site C for this child domain?
2. How do I migrate OU’s from root domain to child domain?
3. I have two exchange servers at site B and site C. How do I transfer mail boxes for to new child domain?