Hi Florent and welcome to the forums!
That’s a good question. You could use something like Gravity Forms or refer to the Woo docs for how create a new field on the checkout page. However, getting that custom data to attach to tickets and display on a per-ticket basis will be pretty tricky to be honest.
We are currently working on a method for handling additional data for tickets, but I’m afraid the feature is not available just yet. If you add your vote to feature, you’ll be notified when progress is made.
Sorry I don’t have a concrete solution for you here, but does this at least help answer your question? Please let me know.
Cheers!
Geoff