Yes, but I am not aware of an existing proxy implementation that would accept JMAP requests and forward these to a CalDAV server, or the other way round.
What I do know for sure is that it‘s feasible for a calendar server to service both JMAP for Calendars and CalDAV. All our work in that regard is open-source in the Cyrus server [1] and is used at Fastmail.
Apart from the exchange protocol one also needs to convert between iCalendar and the data structures used in JMAP. For this, I am currently working on an IETF RFC [2]. Anyone also interested in implementing conversion between iCalendar and JSCalendar, please contact me! We have a test suite for interoperation tests.
