Not quite as easy but basically those social icons are using the font Genericons http://genericons.com/ so if there is an icon there that you want to use, then you will need to create custom code to add more to the style.css for the icon, but also to the theme-customizer.php file if you want this option to show up with the other social icons in the theme options. You will also need to modify the header.php file…. which means editing core theme files. This is where the child theme should be used to do these overrides and modifications.
As for adding something other than another icon to represent your mailchimp, this would actually be a bit more complicated because you would then have to modify the structure of the social bar.
Sounds fun doesn’t it :cheer:
I could do this for you but the bad news (sorry) is that it becomes billable time because it’s custom modifications and not support.