Yes. Just promote the 2008 box to a DC, telling it to be a DC for an existing domain.
Also keep in mind that in Windows Server 2003 and 2008, there is no concept of 'backup domain controller' and 'primary domain controller.' They all can be read and written to at any time (unless the 2008 box is a read-only domain controller).
Re: Windows 2008 Server as a Backup Domain Controller
8 years 1 month ago #35425