class X2AbilityMultiTarget_BlahBlah extends X2AbilityMultiTarget_AllUnits;

function GetMultiTargetOptions(const XComGameState_Ability Ability, out array<AvailableTarget> Targets)
{
    class'X2AbilityMultiTarget_AllUnits'.static.GetMultiTargetOptions(Ability, Targets);

    Targets.Length = Ammo or whatever
}