All FTB packs are based on Forge, which allows for mods to easily be added to the vanilla game. In whatever directory contains your server .jar file, there should also be a 'mods' folder containing a .jar or .zip for each mod you have installed. To add new mods, you'd need to upload the jar/zip for the mod into that folder, as well as install it client-side.
Bukkit plugins are usually serverside-only, and aren't designed to work with Forge. There used to be a program, Cauldron, that tried to allow for Bukkit plugins to work on a Forge server, but it was never really a stable system.
I can't personally think of any faction/economy Forge mods, but that doesn't mean they don't exist. Just keep in mind that (in most cases) if you install a mod on the server, any client that tries to connect will need that mod installed locally, too.
As for making a mod, I understand that Pahimar's Let's Mod series is a rather good introduction.