ZK reference (http://books.zkoss.org/wiki/ZUML%20Reference/ZUML/Processing%20Instructions/taglib/Custom%20Taglib) states:

In addition, you could import all public static methods with an element called import, and the name of EL function will be the same as the method name. For example,

<import>
    <import-name>Labels</import-name>
    <import-class&gt;org.zkoss.util.resource.Labels</import-class&gt;
</import>

However it either does not work, or the documentation needs to be updated with how to use that feature.

See forum post for details and example: http://forum.zkoss.org/question/94785/regarding-custom-taglib-function-import-from-class/