Hello,
I'd like to know if there's a way to make expand/collapse buttons inside k2 articles.

in this case i'd like to have several letters, like for example A, and there would be content underneath it, you would be able to see it by clicking the letter A, and hide the content again by clicking again.

Sorry if it's a dumb question. Looking forward for an answer. Thanks!!

Bump :(

You can use the tabs and sliders plugin available here from Joomlaworks.net. You can also look for something similar at the JED or create your own using jQuery and/or extrafields.