Are you doing extract or all grain?
You can add DME or Belgian candy sugar to the end of the boil, but both methods can thin out the beer at the end. I made a stout that had molasses at the end of the boil that I think turned out pretty good.
If doing all grain, you could look at the grain bill and slightly increase the grain while keeping the water volume the same.
Fast pitch has zero effect on alcohol content. It is designed to make a yeast starter without doing any extra work. You truly need to make more fermentables sugars in your wort for the yeast to eat.