Have you thought about adding a custom link button? Then change the text on the button to "add to calendar". For the link, you'll need to upload a calendar invite .ics (from MS Outlook) file to the Library under Documents (Admin section). Once uploaded, click on the newly created calendar hold icon to get its full file address [see screenshot]. Then go back to your email designer and enter that as the link.
Admin>Libraries>Document Library>your folder(or create one)>Add file Once uploaded: click on icon and scroll to bottom for Reference Link