{template_exists tpl="foobar"}{include "foobar"}{/template_exists}

block.template_exists.php
<?php
    function smarty_block_template_exists($params,$content,&$smarty,&$repeat){
        $template = $params["tpl"];
        if ( $smarty->template_exists($template) ){ return $content; }
        return "";
    }
?>

=================================================================================
{template_exists tpl="foobar" var="doesit"}{if $doesit}{include file="foobar"}{/if}

function.template_exists.php
<?php
    function smarty_function_template_exists($params,&$smarty){
        $template = $params["tpl"];
        $assignTo = $params["var"];
        $smarty->assign($assignTo, $smarty->template_exists($template) ? 1 : 0);
    }
?>