So finaly, when using HR modules, we could not use Timesheet directly with activities (RTT, CP...) created in the HR module.
No, this is the "old way", without HR Module.
With HR module, you must create leave periods through the HR screens (leave periods, calendar)
The only way I found was to create another Administrative project, and create again new activities like RTT, CP. But then in the timesheet, dual leave period appears and does not make sense for user.
Yes, both systems exist to enter leaves : "old one", simple, without validation process, "HR module", more complex to setup butr calculating leave rights and with validation process
Both systems should not be used, you should select one.
Is it possible to link activities defining leave periode from the HR module to the Timesheet?
No, they will appear when leave periods are defined.
Actually, why is the automacally created project from HR module not detected in the Timesheet panel ?
See above.