Hi,
Here is an example for creating contacts, and creating account can be done in the same way.
Please refer to the following steps to create the workflow.
If a contact record is created for an existing lead record, it is created as on-demand workflow. For your case, it can be created as a normal one which starts when "Record is created".
Insert a step to create the related contact for the record.
Set the value of the field in contact record with the existing value which you need in the lead record.
Then update the lead record itself.
Set the contact just created as the parent contact of the lead.
After activating the workflow, you can select multiple / all leads and trigger the workflow for them manually.
This step can be ignored if it is not an on-demand workflow.
It eliminates the challenge of having to create contacts for a large number of leads.
