How to developp bootloader on Arduino nano 33 BLE

Hi, 

I'm looking for a solution to update my program with BLE. I need to do it with a mobile app developped in Flutter. I found the Flutter library to flash the Arduino ( flutter_nordic_dfu ). 

But with the original bootloader, I can't use this library because the BLE is not supported in DFU mode.

I've tried oher bootloader that I found online and the nRF Toolbox application (to be sure it's not coming from my Flutter code) but it failed. 

What can I do to be able to flash the program with BLE ? Is there a way to custom the bootloader and from what files ? I have a J-Link mini EDU to flash Bootloader. 

Thanks !