There are indeed a way to import converted Lead(which is a type of creating converted lead.)
Fields we need for importing converted leads are “ISCONVERTED” , “CONVERTEDCONTACTID” , “CONVERTEDOPPORTUNITYID” and “CONVERTEDACCOUNTID“.
Step 1 : As above fields are not editable, we have to contact Salesforce Support to enable Audit fields. Enabling Audit fields means we can edit few Readonly fields like created date and above lead fields.
Step 2 : Import Account, Contact and Opportunity from Legacy system to Salesforce.
Step 3 : If you imported account, contact and opportunity in Step 2, Salesforce automatically generates Unique ID. We need that unique Id to insert Converted Lead. So Export Account, Contact and Opportunity, which is inserted in Step 2 from legacy System.
Step 4 : Create CSV File with All lead information with ISCONVERTED=TRUE and CONVERTEDCONTACTID, CONVERTEDOPPORTUNITYID, CONVERTEDACCOUNTID. CONVERTEDCONTACTID, CONVERTEDOPPORTUNITYID and CONVERTEDACCOUNTID should correspond to Ids generated by Salesforce for Contact, Opportunity and Account which will be related to converted lead.
Step 5 : Once CSV is properly created with all required Data, Insert it using DataLoader.
Note : We cannot convert existing lead using this process. Leads must be inserted with these four fields. If you try to update lead it will not give you option to edit above fields.
How do you import Converted Lead into Salesforce from Legacy System ?