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.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.