Audacity is a wave editor. Is you have NBTV files, with Audacity you might cut and paste them to an NBTV-program. As far as I know Audacity cannot create NBTV wave files. Gary made PC-programs to generate NBTV video files.
However, to start with: download the file that you find on the bottom of this page:
http://www.nbtv.wyenet.co.uk/sound.htm on the NBTVA website, unzip it and load the .wav file into Audacity. Then you get ideas on how the NBTV signal looks like and sounds like.