The thing about adding extra extract to increase mouthfeel is that the type of extract you use is important in determining the way a beer will feel. Some portion of the extract from each bag or can is unfermentable, there is more unfermentable material in the darker extracts and certain liquid extracts like John Bull are known for not being highly fermentable. Throwing extra light malt extract into the beer isn't going to do as much for your mothfeel, but it will increase your abv as most of the weight of the extra light malt extract is fermentable. The only way you are going to figure out what extracts give you the type of mouthfeel you are looking for is to experiment with the different brands.
Seeing as how you are brewing a stout, if you are not happy with the mouthfeel of your current recipe and it calls for using Light malt extract, you could always swap the light extract out for an amber extract or a dark extract. Adding malto dextrine will work, it is an unfermentable, but remember a little 4 - 8 oz will go a long way. Lactose is an unfermentable sugar it will add a little to the mouthfeel, not as much as the malto dextrine, but the main reason people use this is to add some extra sweetness to a beer.
As was mentioned earlier the best thing to do is to steep some specialty grains that don't have active enzymes i.e. carmel malt, chocolate malt, black patent malt, and then brew, but again you need to realize a small amount of these grains will go a long way.
