Well, first you'd need to make/convert a 3D model for OGRE use, aka making it a .mesh file.
That's the easy part if you have the right programs. Blender's good to use and free, but it's a mess for new users. I, for one, got myself the trials for AC3D, which works very well for this.
As for getting the models in-game, that's the hard part. You'll need to get the meshes the right size for the models, then you have to code them into the item.CFG file, which takes up a considerable amount of time getting it just right.