It will still remain in it's own domain.
You will have 2 domains running on the same wire. One domain with just the
DC and a second domain (the domain you want to add the DC to.)
A DC can not be in two domains at the same time. To add a DC to another
domain you first have to remove it from the domain it is in.(by running
dcpromo and loose all user accounts) then add the server to the new domain
and run dcpromo (picks up the new user info from the first DC) to make it a
DC in the new domain.
Not sure of what you want your final outcome to be, but if you don't want to
remove AD from the server you will have 2 separate domains You can create a
trust between the two domains if necessary.
hth
DDS W 2k MVP MCSE