To begin, make a copy of the WAV file; (right button, drag, select copy here) next right click on the new WAV file and from the context menu select Record.. On the open file; from the Edit menu, select Copy, again click Edit and select Paste Mix... this will double the file size.. repeat this to increase the file size to the desired lenght... then use this file to record (with the red dot button) while you play the original tune to the lenght you want the final WAV file.
To cut the final file to size or to remove the undesired end or begining from the file, stop the slider on the desired position where you want to cut the file, and from the Edit menu, select Delete After Current Position, or select Delete Before Current Position if the undesired portion is at the begining. If you also want to increase or decrease the volume, use the Effects menu, and Select Increase or Decrease Volume. Finally click on the File menu and select Save, or click Save As if you want to save it with a different name and keep the previous copy intact.