For users not using the PortableApps.com Platform, you can add additional fonts to the LibreOfficePortable\Data\fonts folder for use with LibreOffice here. (Do not add fonts to the LibreOfficePortable\App\DefaultData\fonts folder or to the LibreOfficePortable\App\libreoffice\Basis\share\fonts\truetype folder)

Note that any fonts placed here will be available to other applications.  If any other apps (including local apps) are making use of these fonts and don't properly stop when LibreOffice Portable is closed, you may be unable to eject your drive until you close those apps.