Hi @mnio,
HubSpot marketing emails would have to support attachments for that which (unfortunately in this case) they don’t. It’s currently not possible to send actual calendar invites via marketing email.
You’re best option is working with Add to Calendar tools, I prefer this one: Add to Calendar Link Generator | Google Calendar, Outlook, iCal (The links generated by this tool can be modified to include personalization tokens by the way. Pretty neat.)
Best regards!