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 or organization followers and include all the users you want to add as a follower with a comma separated.
3. Now import the Excel file back into Pipedrive like explained here with the using of the ID field:
4. During the mapping of the import make sure you use the deal/person/product or organization follower field on the left side to map it out to your follower column of the spreadsheet