..or any other already enchanted weapons and armor that you find in the game for that matter. For example, when I have the necessary perk to put two enchantments on one item, can I only do it with my own creations or could I also add something like a magicka regen enchantment to the ebony mail?

Thanks.

No, you cant add an enchantment on to an item that already has one, only at creation.

If you first disenchant the gear using Dwemer Detergent http://steamcommunity.com/sharedfiles/filedetails/?id=11843
or maybe the EzE - Weapon and Armor Artifact Disenchanting http://skyrim.nexusmods.com/downloads/file.php?id=8157
then you could re-enchant with two spells. The problem is that learning the enchantment for anything generally destroys
the item, AND then gives you a basic level version of the spell, rather than what you got with the original.

Even if you could, it already has the maximum number of enchantments, two.

Make sure you duplicate it first.