i think you will need trigger for that
try to hide -load unit icon or just make another one icon that will be chain lightning (same range as loading unit range and 0 damage, 1 target)
>units start casting an ability
>>ability being cast equal to (chain)
-change ownership of (target unit of ability being cast) to owner of unit(casting unit)
<you will maybe need to use wait 0.00 sec but try without it first>
set custom value of target unit of ability being cast to (player number of owner of unit (target unit of abiltiy being cast)
order casting unit to load target unit of ability being cast
you will need this with blue letters if you wish to return the real ownership
when units is unloaded
-change his ownership to player index (custom value of that unit)
or go on (load unit ability) and try > targets allowed > enemy , but i don´t believe it will work