A more dextrinous wort/beer can usually be achieved through process/recipe tweaking without having to resort to post-fermentation additions. What makes you think you need more dextrin?
A little goes a long way in terms of adding body to a beer post-fermentation if you think it has a thin/light mouthfeel and want to fill that out. It's useful if you are working with a (typically high-abv) beer that got away from you and thinned out too much and you need to add body back. Also useful in extract recipes if you really want a thick and chewy mouthfeel without residual sweetness. I use it mostly to feed bugs in my long-term aged sours if acidity isn't getting where I want it.
Dextrins are non-fermentable (by beer yeast) carbs. Every beer wort has some. You can increase their proportions through mash parameters, by using malts such as carapils (aka "dextrin malt") or by adding a product like maltodextrin to the boil. Unless you are trying to fix a problem, I would not recommend adding maltodextrin to your fermenter. If you want to experiment with getting higher dextrins (for mouthfeel, foam retention, or whatever), I'd do it in the mash (via parameters and/or malt selection) or boil (with maltodextrin).
