It depends on how sophisticated your new casual is.
If its just an edit of a causal you already have on your server you just need to make changes to itemminicon.ani, mapitemicon.ani, armor.ini, itemtype.dat.
If its a completely new casual then you need to add your texture to \c3\texture\, your mesh to \c3\mesh\, your inventory icon to \data\itemminicon\, and your dropped (isometric) icon to \data\MapItemIcon\ (if you have one). In addition to all the files above you will need to edit 3dtexture.ini and 3dobj.ini as well.
That's all I can think of right now. If you know what your talking about and I missed something please post as I'm still a little rusty atm ;)