Chaptalize is simply the term for adding sugar to raise the alcohol level. Maybe somebody that has made this port can answer for you.
Generally with a port, you make it as a wine, but when the fermentation slows down, you can add sugar to feed the yeast more- raising the alcohol level. Wait to add the bag until we here from someone what the contents would be. If it contains sorbate, that could slow the fermentation down or stop it. Somebody will answer shortly.
Edit: PS see, Wade already answered before I even hit the Post button!