Ajout de champ

Of course you can.
Just just need one project.
But maybe you should use "contact" instead of "customer".
This means you may have one only customer, a "global" one, and each contact will be your customer.

Other idea is to use the Screen Customization plugin to add idClient in screen Ticket.
This way, you will be able to select customer (references in Client object) on the ticket.

With Screen Customization plugin, you can easily add new field.
Just name it idContact and it will work as expected.