I have a feeling this sort of thing would be beyond the mod tool’s scope, but would the following be possible: adding an inventor service to convert redundant resistances on gear into the corresponding %damage? E.g., you have much more than the capped limit of fire resistance, and are using some gloves that provide 15% fire resistance. You take the item to the inventor, pay a Blood of Ch’thon and 5 Aether shards or whatever would be appropriate, and the 15% fire resistance becomes +15% fire damage.
i think that possible if you make several copies each items for replacement. but then it would be many checks, for items. I think better if you make npc who can reroll stats for unique items 
It would be a huge amount of work, bordering on impossible…
The item itself, the prefix and the suffix each provide a fixed set of bonuses. The value is somewhat random, but what the bonuses of each are is fixed.
So you cannot simply replace one stat with another, you would have to replace an item with one that has the same bonuses except for the fire resistance and instead had fire damage - or the prefix with another, or the suffix - or a combination thereof, depending on where your fire resistance comes from (parts could come from all three…)
Yeah, that’s what I thought.