There's no way to mod Armageddon bombardment. It's locked away somwhere....
Have a look in C:\Program Files (x86)\Steam\steamapps\common\Stellaris\events\orbital_bombardment.txt
The damage the bombardment types do can be found in C:\Program Files (x86)\Steam\steamapps\common\Stellaris\common\defines\00_defines.txt
Sadly, the orbital_bombardment.txt is currently bugged, it does not have events set up for the armageddon bombardment so no tile blockers and such will be spawned. It's been posted in bug reports already.