Sinamar is probably the best way.
Alternatively you could figure out how much D-180 Extra Dark Belgian Candi Syrup you would have to use to get your SRMs up... probably about 3/4 lb for 5 gallons or something like that. Then from there, calculate exactly how much water to mix it with to hit exactly your original original gravity, then add it and let it ferment out. This will impact flavor a little bit, but in nothing but a good way I would think.