1. How do I modify an item (YT-2000, Lightsaber, etc.)?
2. If I can't do the modifications myself, which faction must I contract to complete them?
3. Where can I find a list of possible modifications that are available?
1. Items (ships, items, vehicles, etc) are not modifiable at this time (Lightsabers are able to be crafted however, but once must be Force Sensitive first in order to allow that).
2. You can't, no factions can either.
3. You won't, they don't exist.
"I like Justin Beiber Mash-Up's the best" is what I keep telling myself.
And before you begin to point out that people are offering modded ships for sale/trade, modding of ships was done years ago as various rewards. However it has not been done by the admin for over 3 years now, and they will not do so. You will have to wait for R&D and the expected modding system that will follow it.