NT VS Active Dir

i`ve pure NT domain with one pdc & 1 BDC ,now we are planning to add a brand new machine as win2000 & want to implement active directory on it,it is installed as server now when i run DCPROMO & choose “addition dc for existing domian ” as i want to join to old domain it comes up with error domain is not an active direcory domain ,wat i should do to make a part or old (NT) domain so i can run active dir.i want to keep my PDC & BDC as it is .