Hi Zied dams, 
you can try import/export feature of odoo.

Go to contact list view, Then Please select all the Contacts and export them with checking "I want to update data (import-compatible export)" as True. Now you can add company field column like the others (company_id) in it and assign them a value.

Now save and import this file, it will add the company values in all the contacts.

Hope it will help you.