This is possible, but not widely supported enough to warrant use when sending to a 'general audience'. For something like this to work you'd really need to know which ESP (inboxes) your audience uses and cater the css to those clients. There are a few examples of things like this out there on the web, but they're more "bells and whistles" and break more than they don't in the modern environment.
The two ways of going about this for webpages are generally:
1) use a script to create an "accordion" function when you click on something, but scripts will not work (for good reason) in your email inbox for security reasons
2) use CSS to create the accordion. This is more technical to setup and requires bits of css that aren't generally supported in email inboxes (yet). I'm hopeful that someday this'll be a "generally supported" thing but for now it's still a bit "fashion-forward" and won't display for a good chunk of the inboxes out there b/c of the limitations they place on css.
Keep in mind that something like will likely work within the context of the Marketo editor b/c it's web-based (it's a website) rather than "inbox-based" so you've got a more robust set of supported CSS in the editor than you do once the email actually hits your inbox -- don't be fooled by folks promising that this'll work just b/c it's working in Marketo and if you do find something ask for email render test results to ensure that it'll work for the inboxes you're targeting for your specific audience.
Thanks,
Dave
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