There is not a way to tag a recipe in an unindexed book. However, you can have a recipe from an unindexed book show up in your favorites list if you enter it as a Personal Recipe (for more information on how to do this, click here.) You can enter the recipe title, the name of the book it is from in the Source field, and add whichever Bookmarks you would like to it (such as a Favorites Bookmark.)
You also have the option to request to member-index a book if you have the time and desire to do so. Many members have chosen to do this, particularly if a favorite book is not owned by many members and therefore not likely to be indexed soon.