You can also use Windows Live Photo Gallery to add face tagsuse Windows Live Photo Gallery to add face tags to the photos. It will be added as metadata properly and if you don't do anything else then you won't lose any of your other tagging done through LR.
Windows Live Photo Gallery seems to be more metadata friendly than Picasa. Or at least more predictable in what it does (not to mention I prefer the editing wizards a bit and find myself using them over LR sometimes).