How to export and import files related to Task object in Salesforce?

Step 1)  Prepare your import file
Create your CSV import file and include the columns listed below, then relate your activities to your existing Salesforce records:
WHOID (Name) – The record ID of the associated Contact or Lead.
WHATID (Related To) – The record ID of the associated Account, Opportunity, Contract, etc. Objects that allow you to track Activities can be set as the WhatID.
OWNERID (Assigned To) – The record ID of the user that owns this Task or Event.
Status – Only include this column when inserting Tasks. Set value to “Completed” for the record to appear under “Activity History.”
StartDateTime and EndDateTime – Only include these columns when inserting Events. Events with a Start and End date/time in the past are automatically placed into the “Activity History.”
Important: These fields require specific formatting as outlined in our Date and Date/Time value format  documentation. Also, review the special considerations for each field in our Event Object Documentation.
Subject (and optionally Description)
For a full list of available fields, see our SOAP API Developer Guide Event Object documentation and Task Object documentation .

For all of the previous relationships you must use the unique record Id to make a successful association. Because a lookup relationship is being created, simply inputting the record name will not work. You can get the Ids of the associated records by running and then exporting a report that included the Record Id column.

Step 2)  Insert the records using the Data Loader

1. Log in to the Data Loader.
2. Select Insert.
3. Select Tasks or Events accordingly.
4. Locate the CSV file created on step of this process.
5. Click Create or Edit a map and map the Salesforce fields to the columns in your file.
6. Click Next.
7. Click Finish.