Why a repeat calendar erase the actual one?

As a Clerk, I want to be able to repeat a particular schedule for a field ressource.
For exemple, a technician having a schedule for monday to thursday 7am to 5pm, but for the next 3 month, he will work thursday 9am to 7pm.
I select a custom working time and repeat that selection ''weekly'' for thursday.
Result: all the rest of the schedule is erased (monday to wednesday) and thursday get the new schedule?

Is there a way to keep the rest of the schedule?
For now i have to enter day by day (non repeated activity) to avoid the erasing of the rest of the schedule…