We have manually created fields and customized views in our database.
Now we would like to make these customizations in modules in order to be able to update our Odoo again.

Is there a way to create the already created fields in the module if it already exists without losing the data that is already in the database?

Yes, I know, you can also just leave the fields in the database as they are and only adjust the views in the module.
However, the module would not work in another database in this way.