yes you can add it, just get the code from adsense and put it in your post. Adding ads in posts increases the CTR and your earnings will increase
Yes you can do as mentioned by the other friends, but it would be better for the blog if you place it within the main theme, and since you are allowed three ads per page, then all three ads will be shown in all posts and you will have better chance to increase your earnings. Also it will be easier for you to control when you want to do any changes in the ads, you only have to do it in one place instead of trying to remember which post that includes a specific ads. However this depends in your theme and how your single posts displays. Regards,,, *** TheGemini ***