4-5 xotpl/menu.tpl

<li>
    <{includeq file="$xoops_rootpath/themes/$my_theme/xotpl/opt.tpl"}>
    <{if $opt.submenu}>
        <ul>
            <{foreach from=$opt.submenu item=opt}>
                <{includeq file="$xoops_rootpath/themes/$my_theme/xotpl/menu.tpl"}>
            <{/foreach}>
        </ul>
    <{/if}>
</li>