Is it possible to add an instrument to the noteworthy composer software?
No, not really, since NoteWorthy Composer does not contain its own instrument sounds. It can only use the sounds that your computer sound system has available.
But if your sound card is of a type that allows the use of soundfonts (Creative SBLive! family or Audigy family) then you can add or change the available sounds. Similarly, if you have external software such as TiMidity and a suitably fast computer, you might be able to use soundfonts even without a soundfont-based sound card.