It doesn't look like you can change the armor set's abilities at all, however, you can add to it. Also, it seems you would need to need to call IsArmorSet and then UpdateArmorSet so it is easier to do. For example you can do it like this in the global item settings.
Code:
public override void IsArmorSet(Item head, Item body, Item legs)
{
if(head.type == ItemID.SilverHelmet && body.type == ItemID.CopperChainmail && legs.type == ItemID.IronGreaves)
{
return "Random Set";
}
return "";
}
public override void UpdateArmorSet(Player player, string set)
{
if(set == "Random Set")
{
player.noFallDmg = true;
}
}
This is at least my guess. It seems that the way they do Armor sets is different because it is not defined by a group name rather barely defined by their setBonus description (player.setBonus) and the attribute associated with it, so you could try looking at UpdateArmorSets(int i) method in Player class and the setBonus(int i, bool english) method in the Lang class of Terraria.