Its possible, use a trigger to keep track of every level the hero has and add a hidden ability with the damage bonus, just update the ability whenever the damage is increased. In whole numbers the game can deal fractional damage, after armor and whatnot, but only whole numbers are displayed or subtracted. I think the game just drops all extra numbers. If you want to modify the base damage, white numbers rather than green +s you'll need to use upgrades but the process is the same.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory