First, welcome to HBT!
I suspect you don't need to boost this recipe at all. As often happens, your OG is artificially low due to insufficient mixing of the hot wort and cold water in the fermenter. Of course, this assumes an malt-extract wort and partial boil. Please post the recipe and general procedures you used.
Boosting OG and therefore potential alcohol is as simple as adding more sugars for the yeast to eat. For a high-quality beer, that generally means malt-based sugars, like more malt extract. Cane or corn sugars can be used in moderation to increase alcohol without increasing the beer's body.
As for creamy mouthfeel, the nitrogen has something to do with it, yes. The flaked unmalted barley used in large proportions in Dry Irish Stout also has a lot to do with it.