Here's the thing with fixing a stuck fermentation. If it is stuck because of a lack of nutrients or, more usually, oxygen, pitching more beer yeast is unlikely to help much. The champagne yeast can handle low nutrients, low oxygen, and high alcohol, but will only ferment small sugars. It will not metabolize longer chain sugars, but neither will most beer yeast (though beer yeast will ferment some of the maltotriose, which the champagne yeast will not).
It is possible that your fermentation is not stuck, but done (though 1.028 does seem high, unless you're brewing a barley wine or other big beer). I would recommend the following:
1) rack the beer to a another vessel - this will stir up the yeast and introduce some oxygen (you don't want too much though at this point as you do not want to overly oxidize the alcohol, which can cause off-flavors)
2) increase the temperature
3) wait a day or two to see if anything is going on...if not try pitching some properly rehydrated dry yeast
EDIT: just noticed you said it has been fermenting at 70-78F. First, this is really too warm, unless you're brewing a saison or something where high fermentation temps are desired. Second, I'm not sure now if warming the beer up per my suggestion above is feasible or even worth it at this point. I'd just rack and see what happens.
