Unfortunately, no. (This is Microsoft's big advertisement for putting in Windows 2000. All machines can be standalone or domain controllers with no rebuilds!) In the NT 4 environment, you can have either a domain controller or member server and this is set during installation. (I've heard of various hacking methods where a domain controller can be made into a member server and vice-versa, but this is not supported by Microsoft and may irrepairably damage any data on the machine.)
The best way to do this is build a machine to be a BDC, add it to your domain, promote it to PDC, then remove the other machine (after transferring any files off).
Hope that helps -