Is it possible to add more material palettes to the material editor once all 24 slots are used up?
The 24 slots that are available are intended to be used as areas to edit your materials. Once you are happy with the material and you have applied it to your geometry, you can then save the material to your material library, and clear the slot to then modify another material. To create and save to a library, open up the Material//Map browser from the rendering pulldown menu, (choose browse from material library) and then save as to a different name. This process is then repeated until you have all the materials you require.