I don't think you can attached related items to a converted lead. When a lead gets converted all the Attachments and activities get transferred to the Account and/or contact. Since you are importing, do you have an equivalent Account or Contact for the converted lead? You import the activities and notes and attachments to those records instead.
Or alternatively, create the lead and then manually convert it.
"All notes and attachments from the lead are converted and attached to the new account and contact. All open activities and activity history from the lead are converted and attached to the new account, contact, and opportunity. The converted lead record can no longer be viewed, although it does contribute data to reports."