Yes, the life_leech_damage_modifier can be applied to an entity or squad, to increase its life leeching with each attack.
Look into modifier_apply_ext/modifiers/modifier_03 of the Data\attrib\ebps\races\dark_eldar\troops\dark_eldar_leader_incubus.rgd entity, for an example of its workings.
In this case, the presence of each of the incubi, increases the life leech damage of the whole squad! That is why the squad is difficult to take down :twisted:
Now, life leech, does not increase Maximum Health of t he entity that leaches damage, but it rather replenishes its health with each attack it scores (proportionate to damage caused and life leech modifier).