There is a conversion script which you can use to convert XScreenSaver
screensavers to the gnome-screensaver format.  The conversion script is located
in the documentation directory (/usr/share/doc/${PF}/).

The migrate-xscreensaver-config.sh and xscreensaver-config.xsl files will be
needed to convert the screensaver.  Make sure to enable the executable bit on
the script (chmod +x migrate-xscreensaver-config.sh).

The script takes the module to convert and creates a .desktop file corresponding
to the module.

migrate-xscreensaver-config.sh /usr/share/xscreensaver/config/*.xml

The above command should convert all xscreensaver modules.

You must copy the desktop files into /usr/share/gnome-screensaver/themes (cp
*.desktop /usr/share/gnome-screensaver/themes), and the executables into
/usr/libexec/gnome-screensaver.

Going to the gnome-screensaver-preferences dialog should now show the new
modules in the list.