Adding a staff to an existing score will depend on which version of Finale you are using, check the Help Guide for more information about this process.
To get an existing non-template score to use the VDL sounds, you will need to have installed the XML files (Percussion MIDI Maps) and then load the FIN_VDL Percussion Library to the score. At this point, again, how you procedd will depend upon which version of Finale you are using.