I'm afraid that's not possible....if Lightroom hasn't detected a face, there's nothing you can do to add that face to the People section. However, if Lightroom HAS detected a face, but it doesn't appear in the People section as an unnamed face, it's likely that Lightroom has detected less than 5 instances of that face....basically an unnamed face will only appear in the People section when Lightroom sees it in 5 or more images. In that situation, however, the face region WILL apopear in the keywords panel of the image (top of the panel above the keyword section). If it's there, hovering over the face icon will show the rectangle over the matching face in the image. You can also add the appropriate name to the tag in the panel, and once a name has been added it WILL then appear in the People view, even if less there are less than 5 instances in the library.
That is why this was a feature request. We need a future to tag people in the pictures. Tagging is a very important organizing too and when you can not tag a missed person in a picture then that makes the whole people tab incomplete and not very useful.