it depends on what version of windows server you are using. I know that with Windows Server 2003 you can use ADSIEDIT.mmc to edit the schema of active directory. you can get it from Microsoft.
**** BUT **** be forewarned!! making one mistake with this program can hose your entire network since any changes made will replicate across your domain. please read up on editing the schema on Microsoft’s website and be very careful with what you are doing. there, you have been warned.