Is it possible to modify existing Nordic firmware source and upload it over OTA using own generated private key?

I have a Thingy52, but no DK. I would like to know if it is today possible to upload a custom firmware only using OTA (no DK), eventually starting from the Nordic Thingy app firmware source?

I asked this, because I managed to create a dfu package using nrfutil, starting form Nordic Thingy app source(using keil to compile), but I always get an INVALID OBJECT error message when trying to perform OTA suing nrfConnect app, while it works successfully using the zip file provided by Nordic ( 

I'm afraid the problem is that I'm compiling with my a generated private which is different from the one used by Nordic to compile the bootloader.

Is it possible to perform DFU on devices implementing the Secure DFU by using a new private key?

Can Nordic release a dfu zip package to unlock Thingy devices to receive custom firmware with new private key files?

Thanks in advance!