You can save/restore them manually by selecting search results to go to an editor window, then saving the buffer as a file. When you restore the workspace you can copy the contents of the file into the search results window. They may not be valid though, if the files have changed in the meantime. You can probably automate this with a macro if you're really keen but why not just run the search again?
In the find dialog, you can use F7/F8 to step through previous searches that you've done and if you right click in the dialog you'll see you can also save particular search settings with a name.