I think you can't do it exactly like you want, because damage is counted using hero damage base, number of dice, sides of per die and strenght. Only way is to make an upgrade that will increase damage base or number of dice and give it to hero.
Use upgrades. Make it a damage dice bonus or so upgrade, but which applies to a class that your hero doesn't have. Example: If your hero is melee make it apply to Ranged class. So it won't display as an upgrade, but will upgrade the damage of your hero. For more informations, read my Tutorial.
Although it doesn't contain such an example, it will tell you what are Classes and stuff.
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.