Taking free sign language classes offered in my town, can I write that as an EC on my app?
Yes, you can include it. Unless you're getting fairly "fluent" and using it regularly, you might use the Hobbies designation and group it with other leisure-time activities.
If you have acquired some skill, you can list it among your Languages. You'd be asked to check one of the following on the AMCAS application:
Basic I speak the language imperfectly and only to a limited degree and in limited situations. I have difficulty in or understanding extended conversations.
Fair I speak and understand well enough to have extended conversations about current events, work, family, or personal life. Native speakers notice many errors in my speech or my understanding.
Good I speak well enough to participate in most conversations. Native speakers notice some errors in my speech or my understanding, but my errors rarely cause misunderstanding.
Advanced I speak very accurately, and I understand other speakers very accurately. Native speakers have no problem understanding me, but they probably perceive that I am not a native speaker.
Native/functionally native I converse easily and accurately in all types of situations. Native speakers may think that I am a native speaker, too.