Thanks for your answer, the problem I saw with that approach is that the ergonomic will be changed significantly :(

I agree with you. The other solution would be to put all the task fields readonly while editing the task and adding task work.

How this can be done?

Here is the documentation for menu: http://doc.openerp.com/trunk/developers/server/03_module_dev_04/

for view: http://doc.openerp.com/trunk/developers/server/03_module_dev_03/

Thanks Maxime, with that I solved my first problem (see How can we block the editing of planned hours in a task), I will post my implantation for this question later this week.