Exchange 5.5 had its own directory, along with its own directory import and export functions. Exchange 2000 uses Active Directory as its directory and directory service, so what you're really asking is how to import data into Active Directory. There are a couple options available to you. I would start by reading the Step-by-Step Guide to Bulk Import and Export to Active Directory. Since you have experience with CSV files, you might prefer to use CSVDE instead. Details on using this tool to bulk import contact data into Active Directory can be found here.
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