This is a Playnite plugin that gives you an option to download game information from Steam store even for games that are not imported from Steam.
When downloading metadata while editing game, it will give you an option to specifically choose what store page to use as metadata source. When downloading metdata in bulk, it will try and match games automatically (it's not and can't be 100% accurate).
Check releases page.
Follow instructions from Playnite's wiki page. Basically extract downloaded zip file to correct folder.
After installation new Steam Store metadata source will be available on metadata download dialogs.
If you have some issues then please open new issues item.