No, you don't need to do it programmatically nor should you code against the blog or any other feature.
I told you look in the settings for the blog and in the settings for the feed manager. If you look you will see the settings, then try them.
You can enable excerpts in the blog and you can enable excerpts within the feed manager so you have 2 ways of doing it.
If you look at demo.mojoportal.com right now, I have create 2 blogs and on the home page I've created 2 feed managers.
The first feed manager is using Excerpt mode in the feed manager and consuming the feed from blog 1
The second feed manager is not using Excerpt mode but it is consuming the feed from blog 2 and blog 2 is configured for excerpt mode.
When the blog is configured for excerpt mode then you will see a new tab in the blog editor page where you can edit the excerpt.