Debugging a Nordic MCU (feather NRF52) using Segger J-Link
I haven't tried this, but OpenOCD supports both the nRF52 and the JLink.  Something like (on Linux):

openocd -f /usr/share/openocd/scripts/interface/jlink.cfg -f /usr/share/openocd/scripts/target/nrf52.cfg


might start OpenOCD.

You can then connect with (nRF52 flavour of) gdb (arm-none-eabi-gdb). 

You can find information about using gdb with OpenOCD here and here.