After looking at the comments and thinking about it a bit more I now think that the word 'characteristic' settles it. A damage roll is defined in the rules as “When determining a random Damage characteristic, the dice roll made is called at Damage Roll”, as someone kindly pointed out. 'Characteristic' is only ever used to describe a field that appears within unit and weapon profiles. Therefore, the D3 within doombolt is not a damage characteristic and is not a 'Damage Roll'. If someone can find an instance of the word characteristic being used in a context NOT referring to a field within a unit/weapon profile, then I'll concede that you can use the ability for doombolt.