Easiest thing to do would be to render the object into a render texture and drag the render texture (using UITexture).
If you really want to drag the 3D object itself, you will need to change its material to use a render queue that's higher than NGUI. NGUI by default uses render queues 3000 and up (+1 per each new draw call), although you can also set this value explicitly on panels.
P.S. I am assuming a 3D UI drawn using the game's camera here. If not, then you're in for more fun -- as NGUI is generally drawn using a separate camera, and you can't simply move one object from one camera to another. You'd have to mess with its layers, and ensure that the object appears in the same position, etc... it's much more difficult.
