Well, if you are using tag files instead of TLD/Facelet libs then you can't import it to JBoss Tools Pallete but you can add your tags manually. See http://docs.jboss.org/tools/3.1.0.GA/en/jsf/html/palette.html#PaletteEditor
But this won't make the visual editor render your tags from Pallete. You will have the DnD feature available only. Please try http://docs.jboss.org/tools/3.1.0.GA/en/jsf/html_single/index.html#vpe_templating for your case.