Hey there!
For custom properties or data, I typically use the export/import feature to mass update any data. One option is that you can pull the company ID that you’d like assigned to each contact/email and copy it to all of the email addresses you have in an excel spreadsheet.
This works if you have contacts already created by exporting the list of contacts you need to assign a company to OR you can start fresh and have the contacts created upon upload.
Hope this helps!
Brian Carter