Can one make a bracha of לישב בסוכה if one is already in the middle of his meal? Rav Ovadiah Yosef deals with this question and comes out with a very interesting psak.
With regards to the main question, even though the mitzva of sitting in the succah includes all types of actions, since we have established that the bracha is said only when one eats there, then once one has concluded his meal he can no longer make the bracha. However, if one is still in the middle of his meal, then even though ברכות המצוה generally need to be said עובר לעשייתן - before the performance of the mitzva - nevertheless Rambam rules (Hil. Brachot 11:5) that one can still make the bracha so long as he is still in the midst of performing the mitzva.
Rav Yosef moves from there to note that even if one is no longer eating bread, he can still make the bracha even if he is eating foods that would not require the bracha if they were eaten independently, such as fruits. The rationale for this is that since they have become part of the meal, they can be used as a lever for making the bracha on the entire meal.
Rav Yosef then makes an intriguing move. Normally the halacha is that one does not need to make a bracha on certain foods since they are normally eaten in an עראי (casual) manner. However, if one were to be eating such foods in the middle of a meal, then the eating of those foods, even including a drink of water, would be transformed into אכילת קבע, a more established form of eating, and thus it would be forbidden to eat any part of the meal outside of the succah.