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: Fruitcake Old Ale
Primary: Simple Cyser '15
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider (2014 & 2015)
Planned: So many ideas, not enough time to brew them....