It's not required to install hardware packages via Boards Manager. That's just a way to make installation easier for the users. For development and testing I recommend doing a manual hardware package installation instead. You can do this by copying the package to the {sketchbook folder}/hardware folder (if the hardware folder doesn't exist then create it. You can find the location of the sketchbook folder in the Arduino IDE at File > Preferences > Sketchbook location. Restart the Arduino IDE if it's running and the boards should now appear in the Tools > Board menu.
Let's say your hardware package vendor name is zvipesh, I know the architecture is AVR because you said they are ATmegas. The folder structure should look like this:
{sketchbook folder}
|_hardware
|_zvipesh
|_avr
|_boards.txt
Note that this folder structure differs from what you will find with the hardware packages installed via Boards Manager in that there is no version folder.