Q: "How can I migrate converted Leads into Salesforce?"

A: Like the tip from yesterday (on assigning records to inactive users), this one requires (1) that the "Create Audit Fields" permission is enabled--contact salesforce.com Support to activate it--and (2) that you migrate the Leads through the API (with a tool like Data Loader).

This can only be done when inserting new records, and you'll also need to provide values for the fields "ConvertedAccountId" and "ConvertedDate" (and optionally, for "ConvertedContactId" and "ConvertedOpportunityId").  For the full writeup, see http://tinyurl.com/MigratingConvertedLeads