Do you mean body as in mouthfeel, or body as in gravity? If you’re looking for increased gravity you can just add some DME to the wort. Of course, this will give you a higher percentage of alcohol after fermentation as well. If you’re looking for a smoother, heavier mouthfeel, you can steep some carapils (as crushed grain) for 20-30 minutes and include the resulting “tea” in the wort volume. I usually use 250-300grams in a 5-6 gallon batch, steeped at 65-70C. This will enhance the mouthfeel and head retention but it won’t affect the flavor or level of alcohol, since most of the carbohydrates are non-fermentable.