Not even that difficult!
You can do this all in the CS.
edit: ninja-ed by hollaajith - plus I misread. You can swap steel with ebony for everything I said below...
If you want to affect ALL steel longswords, just open up the steel longsword in the CS. Make sure that you have the CS disc in your disc drive. Then, where it shows the model being used, something like "steel_lonsword.nif", click on that. Then go to the CS disc and find the model for the ebony longsword in the Meshes/w/ sub folder. If you have difficulty finding it, just open up the ebony longsword in the CS and see what the NIF name is. Select the model and save the steel longsword.
If you want to make a new weapon, use the exact same process as above, but change the weapon ID before closing the window. And when it asks if you want to make a new object, say yes. Then you can either add that new weapon to your inventory directly player->additem "YOUR ID" 1
Or place the weapon in the world yourself.
I hope that helps. I'm doing this all from memory, so I'm sorry if I am not being very specific.