First make sure keep 2003 domain level as it the DC as is….
Now add Windows 7 computer to domain as GPO management. This helps to enable to configure Win 7 specific settings and it is recommended, that to create separate GPOs for Win 7 only. It is possible user filtering with RSAT feature installed in a Windows Server 2003 R2 domain / Windows 7.
Remember, you will not be able to manage these / settings by using a Windows 2003 GPMC. Before deploy GPO to Windows 7, it is needed to copy all ADMX / ADML files from Windows 7 to the DC.
See this link, which asked earlier by a member “Windows 7 clients on windows server 2003” has almost the same and the approved answer.
Also link here for Deployment.
Hope it may help you!