Well, there are separate ways to do that.
Method 1: You copy each armor's record with your own ID. That is, you launch CS with your plugin as active and another plugin as inactive; then you edit each armor's record and add your prefix to the ID (or replace their prefix with yours). When CS asks you whether you want to create a *new* record, answer "yes". You'll have to pack all meshes and textures of those armors you want in your mod into its distribute archive, though. Advantage is, the player won't have to have other plugin installed to play yours.
Method 2: You launch CS the same way, but you *don't* change anything in edited records, simply hit "Save" on each armor. This way, you don't have to distribute other mod's meshes and textures, but you must warn the player in the readme, in the forum thread, and whatever, that they MUST have that other mod to play yours and that the mod *WON'T* work otherwise.
Method 3: This one is unconventional; I used it in my Almilia's Daughters mod (link in my sig). It's best used when you simply want NPCs to wear some flashy modded things and not a custom armor given to the PC. You'd have to be really well-versed with CS to use it, though. In essence: it uses a compatibility patch that detects which mods are present in player's game and which are not, and from that it decides (or allows a player to decide) which articles should be given to each NPC. Pros: you neither have to have any meshes packed in your mod nor enforce the player to have other mods installed; you simply use those mods if they are there and do without if they are not. Also, it makes your mod versatile, allowing to take advantage of multiple mods. Contras: it breaks immersion, since you have to activate the script somehow (through an amulet or something) and answer questions in the dialogue mode before armors will be distributed; this method won't work automatically.