Can I manipulate values for priority and changefreq?
Yes and No. This plugin has no options page so there is no way to manually set the priority of urls in the sitemap. But there is automatic post priority calculation based on post modifaction date and comment activity, that can either make post priority go to 100% (1.0) for posts with many and recent comments or 0% (0) for the oldest posts with no comments. This feature can be used to your advantage: by re-saving your most important older posts from time to time, keeping the lastmod date fairly recent, you can ensure a priority of at least 80% (0.8) for those URLs. And if you have enough comments on on those pages, the priority can even go up to 100% (1.0). If you cannot live with these rules, edit the values $min_priority, $max_priority and $frontpage_priority in xml-sitemap-feed/feed-sitemap.php but be careful to NOT do an automatic upgrade or it will overwrite your customisation.