Hello @remibusseuil,
In order to add new FLASH, you can follow two ways:
1. Via MCU settings:
Create a new RAM/FLASH in MCU settings, properties->C/C++ build->MCU settings:

Click on Refresh MCU cache:

Click on apply and close:

2. Click on Edit memory in Project settings->memory->Edit memory:

Add a new FLASH and press OK :

Finally, are you currently experiencing any other problems?
If the response is yes, do not hesitate to let me know
BR
Habib.