Add Target always adds a single extra target within the range of your attack to the attack, and this target cannot be an enemy already targeted by the attack (a single attack ability cannot target the same enemy multiple times). So if you got an Add Target effect on an area attack, you could pick one enemy within range but outside of the area (because all enemies in the area are already getting attacked) to get hit by the attack as well. All conditions and other effects of the attack (other than effects that would add additional attack targets) would apply to the added target, as well. Each instance of the Add Target effect adds an additional target to the attack using the above guidelines. Add Target DOES NOT add a hex to the area so effects that apply to the "Area" don't apply to the Add Target.

Note that if an attack ability targets everything within a specified range, Add Target would do nothing since you are already targeting everything in that range.