Hi. Variable you're looking for is probably $this->metakeySource.
But note that $this->getMetaTags() function (see source in sef.ext.php) automatically generate meta keys from $this->metakeySource variable (if not set, from $this->metadesc). It's only "source".
If you want to create meta keywords on you own, you must edit directly array produced by that function like that:
$meta = $this->getMetaTags();
$meta['metakey']="your_own_keywords";
$newUri = JoomSEF::_sefGetLocation($uri, $title, @$task, @$limit, @$limitstart, @$lang, $this->nonSefVars, $this->ignoreVars, $meta, ...);