first Composer

public void onClick$YourSelectedUser() {
        User user = // retrieve your user;
    	Map<String, Object> map = new HashMap<String, Object>();
    	if (user != null) {
    	    map.put("User", user);
    	}
    	Window win = (Window) Executions.createComponents("yourNewWindow.zul", null, map);
        win.setModal(true);
    }

youNewWindow Composer:

public class UserFormComposer extends GenericForwardComposer {
    @Override
    public void doAfterCompose(Component comp) throws Exception {
        super.doAfterCompose(comp);
        User user = (String) arg.get("User");
    }
}