AFAIK the nRF52840 doesn’t come with a bootloader preprogrammed. So while you can flash a bootloader and then use that for future firmware updates, you need to connect via the SWD debug interface at least once to initially flash the bootloader. So if it is just you using your board you could simplify things and skip the bootloader.
So you need an SWD interface.
For the BusPirate there is for example the ported pico debug probe firmware, see Porting Pico Debug Probe . But I’m not sure if that already runs on a BP6, I think it is just ported to the BP5 yet.
Once you got that running you can use OpenOCD to connect to the nRF52840 SWD interface. Once the debug probe firmware runs on the BusPirate, it works just as any other CMSIS-DAP probe. So you should be able to follow other guides for using OpenOCD to flash nRF52840.