More yeast won't help a stuck fermentation caused by incomplete conversion. Amylase enzyme is the only readily available product that will break down starches and complex sugars so that they can be fermented. Unfortunately, amylase enzyme is nearly impossible to stop when added to the fermenter. There are a few possible fixes:
Gently stir the beer and see if you can rouse the yeast back into action. Perhaps conversion was more complete than you thought, and the yeast just pooped out early. Along the same lines (but more expensive), adding a big starter or another packet of dry yeast may help.
If conversion was indeed incomplete, you are in a bind. Beano (the anti-gas pill) will add amylase enzyme to break down the complex sugars. Unfortunately, unless you stop the yeast when the desired FG is reached, the beer will finish VERY dry.
To stop the yeast you have two options: pasteurization, which will denature the enzymes and also kill the yeast, or sulfates (Campden tablets).
To pasteurize, bring the beer up to 170° for about 10 minutes, then chill it again. Avoid aeration. When bottling you'll need to add both yeast and priming sugar.
To sulfate, add one crushed Campden tablet per gallon of beer. With this option, you'll need to force carbonate.
One last option when using Beano in the fermenter is to just let it finish dry, then backsweeten to taste with lactose or Splenda. Bottle as usual.
