In trying to get FTK to create a PST from email items I'm finding, it will only spit out a folder full of files and folders.
After searching for and finding the items you wish to put into a PST, select (checkmark) all the items in the file list.
Right-click the file list and select "Create Bookmark."
On the Create New Bookmark screen, choose "All Checked."
Make sure "Parent Email" is selected.
Right-click the file list and select "Export..."
Make sure "Export emails to PST" is selected
FTK can only generate a PST file from actual emails. However, while doing index searches it's possible to find many "pieces" of emails that do not contain it's parent (the email object). By selecting "parent email" when creating a bookmark, it forces all the actual email parent object to be in your list as well. This way the PST can be created.