there should be a way with just the data editor, but a trigger can work if you just want to test it
make a custom damage effect in the data editor just for triggered damage
then you can change the damage value of this effect and apply it in any trigger.
So you can have a generic "unit takes damage trigger" with the actions:
- set effect value for custom damage effect to (real-damage taken)
- apply effect custom damage effect to (other unit)
be careful though to make sure you don't have an infinite loop because the custom damage effect will also trigger the "unit takes damage" event, so you would need to check the damage source too, if this is an ability that will be on multiple units