You can use this following solution to get this done:
1. Add the ID column to the deal list view and export out the list to Excel like shown here.
2. In Excel add a column for deal/person/product and/or organization followers and include all the users you want to add as a follower with a comma separated in the cell.
3. Now import the Excel file back into Pipedrive like explained here with the use of the Pipedrive system ID field.
4. During step 2 of the import, the mapping stage, make sure you use the deal/person/product and/or organization follower field on the left side to map it out to your follower column of the spreadsheet.
With these steps, you will updated your current items in Pipedrive and adding followers in bulk.