It's been quite awhile since I've touched any co files, so not sure how much help I'll be. Anyway, to get started on this, adding an item can be quite simple, or perhaps quite hard, depending on how newer clients are constructed, and whether or not the client needs data to be recompiled into the c3.wdf or not.
Firstly, to add a new item, you need to decrypt the itemtype.dat (can't remember name of tool) then add the new items details, and convert file back to .dat
If you're planning to make a custom graphic for the item that does not currently exist within the client, simply create it in photoshop, making sure you've got your alpha down, save as .dds, and toss the image files into the following folders within the clients data folder.
-ItemMinIcon
-MapItemIcon
Then to ensure your items read the correct images, you'll need to make the appropriate changes to the ItemMinIcon.ani and MapItemIcon.ani in the ani folder, which should be capable of opening directly with notepad.
Aside from all that, in order to give an item a specific functionality, you'll need to script the right click function for it in your source coding yourself.
Hope that helps, goodluck.
Edit:
To Add Items to shops, it's shop.dat, lol, but anyway, here is a guide from someone else on how to perform, aswell as a decryptor to help with the work at hand.
