If you're sure it's just an attenuation/fermentability problem, and not just a stuck fermentation, adding a fully fermentable sugar to up the gravity and lower the apparent mouthfeel is probably the best way to go.
Packaged: 15 Minute APA, Simple Cyser '14 (Nelson Sauvin Dry Hopped!), Spiced Cider, Spur of the Moment Graff
Primary: Simple Cyser '15
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider (2014 & 2015)
Planned: Fruitcake Old Ale, Not Just for Breakfast Stout. Otherwise, so many ideas, not enough time to brew them....