It s possible to add a field in a product. A field add but not in users base, but in product Good day Frederic the frenchy.
It is planned for 4.2.0, right now you can add a field, but it cannot be used anyhow. Please describe me your task, and I'll try to suggest a solution.
in a product, i have neeed a field for save a information for each product (for exemple number of my paper book)
Yes, that is exactly what planned for 4.2.0. Right now this behavior can be implemented by hardcoding table: product_id# => book# somewhere in your custom code.