Hello,
Thanks for reaching out and for your interest in CryptPad!
To my knowledge Proton aren't using any standards, only open sourcing little parts of their work. See for example where lies the code source of their calendar web interface: https://github.com/ProtonMail/WebClients/tree/main/applications/calendar.
With so little information at our disposal, without even talking about the lack of resources (funding and time) from us, it seems very unrealistic that we can make CryptPad & Proton Calendars interoperable.