Well install it first.
# urpmi task-mate
Then in ‘/usr/share/xsessions’ I create a file ‘mate.desktop’ with:
[Desktop Entry] Type=Application Exec=startmate TryExec=mate-session Name=Mate Desktop Comment=Lightweight GTK Desktop
and it will open from sddm login screen as any other desktop. Don’t know if this is the best way, if there are improvements/enhancements/betterway let me know.
Edit: Had to force install of ‘libwnck3-3.14.0-1-omv2015.0.x86_64’.