I believe you can use IsolatedStorage to save your pre-defined bookmarks. If you use the same key "ESRI.ArcGIS.Client.Toolkit.BookMarks" and store the same object MapBookmark.
You can look at LoadBookmarks() and SaveBookmarks() methods when you download source code for Bookmark from the Toolkit in CodePlex http://esrisilverlight.codeplex.com/ to have an understanding of what you need to do in order to populate your pre-defined bookmarks.
