Not aware of any way to load new sound banks into TTS-1. I think it is closed, and intended to provide GM compatible sounds only.
For a multitimbral synth with more sound options, take a look at Sampletank 3 CS, or Kontakt Player. Both are free VST plugins and come with sample sound banks to try out (but not GM). They both offer upgrade paths to full versions if you like.
TTS-1 is not a VST that you can just use in another DAW as is. It is a DXi format instrument, and the referenced DXi wrapper technology lets it emulate a VST so you can use it in another DAW instead of Sonar as a VST plugin.
Most commercial soft synths do not have GM soundbanks, so TTS-1 is still useful for that.