Flame Wall projectile scaling

How do you scale the non-dps, on hit damage that flame wall provides to projectiles? I can't find the information; I did read a post saying support gems don't work, but that doesn't seem right...

"

Any projectiles fired through the wall by you and allies deal added fire damage and apply the wall's secondary debuff on hit.

If I got you right, you're asking about the non-DoT part.

The way it works is the same as with + flat added fire damage item mods. E.g. you cast Frostbolt, after collision with the wall, it gets +# added fire damage, which is scaled by supports linked to Frostbolt (like its base cold damage). Flame Wall's supports won't affect it indeed.

So you're saying the only way to scale the damage is to modify the projectile skill that passes through the wall? Then there are no support gems that buff flame wall's flat damage directly? Is there is no way to scale it independently?