so, what you want is that you can drain life into ally or drain life from ally?
if you want to drain life into an ally, you can tick that in the map options somewhere. I think you go advanced, gameplay constants, and go to the abilities. You will see a lot of abilities there,
you will see divine sheild can be turned off,
avatar can be turned of
invisible units bestow auras
and one of the choices
mana siphon drain to ally, by default ticked True or Yes
and lifedrain drain to an ally, which is by default ticked False>>>>> this is what you want to change
or if you want to drain the life of the ally? well Im not sure,
if you can make it transfer negative HP to allies then that is draining from them. but can you put negative value in the transfer amount?