You need to eval the template somewhere to be able to use it. You can use a plugin to do it but pick a hook that is evaled prior to the other template you are calling this new template in (I called the template "servery_template' in this example).
PHP Code:
eval('$serwery .= "' . fetch_template('serwery_template') . '" . ", ";');
You'll also have to make sure you cache the template (hook cache_templates):
PHP Code:
$globaltemplates[] = 'serwery_template' ;