Off the top of my head, I don't think there's a way to give a unit health regen that isn't a percentage increase (I'd have to test that theory though).
Ways to do it:
1. Have the upgrade change all buildings of that type into an exact replica of that building, except, it has the desired hp regen.
2. Give the building an ability which gives it hp regen (like unholy aura) but make it so the hp regen is not percentage based (many abilities with hp regen like healing fountain aura and unholy aura have that option). I think you're limited to 9 hp/sec though, on those abilities. Have the upgrade increase the ability if you need multiple leves of hp regen.
3. Self-castable only rejuvenate/unholy frenzy buff, or perhaps a dummy unit that permanently buffs all towers of specific type with it.
Just some ideas. But I'm a little hesitant to say that it's impossible w/ upgrades.