Do you have active directory established in the native mode? If so, once you've created the BDC's computer account in the active directory structure, and you've logged onto the BDC, you should be able to use Active Directory setup to add the BDC tothe existing domain. I ran into a simialar problem when I tried to add a BDC to my ADS domain.
To change the domain mode (this is right out of the windows Help Utility)
Open Active Directory Domains and Trusts.
Right-click the domain node for the domain you want to administer, and then click Properties.
On the General tab, click Change Mode, and then click Yes.
Caution
Do not change domain mode if you have or will have any Windows NT 4.0 domain controllers. You can only change the mode from mixed mode to native mode. Once a domain is running in native mode, it cannot be changed back to mixed mode.
Notes
To open Active Directory Domains and Trusts, click Start, point to Programs, point to Administrative Tools, and then click Active Directory Domains and Trusts.
For more information about domain modes, see Related Topics.
Hope this helps.
Mike