For SQL Server 7.0, you can use License Manager.
This is the step by step description of adding licenses on Windows NT:
Logon to the NT Server and run License Manager.
(Start -> Programs -> Administrative Tools -> License Manager).
Click on the Products View tab and choose "Microsoft SQL Server 7.0".
Click Add licenses button to add more licenses.
For SQL Server 2000 you can use SQL Server 2000 Licensing Setup utility.
This is from SQL Server Books Online:
You can add device or processor licenses after installing SQL Server,
using the SQL Server 2000 Licensing Setup utility in Control Panel.
(Do not confuse this licensing utility with the Windows Licensing utility,
also found in Control Panel.)