Hi Philippe.
I would suggest you refer to AN4282, Using the Kinetis Family Enhanced EEPROM Functionality
https://www.nxp.com/docs/en/application-note/AN4282.pdf
and check the section 3.1 FlexMemory partitioning.
If you want to repartition the EEPROM, it is said you can mass erase the chip FlexNVM, and do the EEPROM partitioning.
Regards
Daniel