If you already have Contacts or Organizations in your company Pipedrive account and wish to add activities for them in bulk, we suggest utilizing the Pipedrive System ID function when importing a spreadsheet. 

To acquire a spreadsheet containing the necessary Person or Organization IDs, we suggest going to the List View and using the gear icon to make the appropriate Pipedrive System ID field visible. Once that field is visible, export the filter results to a spreadsheet, provide the necessary activity information, and import that spreadsheet back into your Pipedrive account.

For more information on how to import data into Pipedrive with spreadsheets read this article.