No, ClarisWorks cannot build a custom 256-color palette based on the colors in a document.
ClarisWorks does allow you to choose between the Claris 81-color palette and a variety of 256-color palettes included with ClarisWorks. You can also create your own palette manually by dragging the palette windoid out into the page and double clicking colors you want to change in the palette windoid, and then going into Preferences and saving the palette.