Created a user in SuiteCRM and added a IMAP connection to their mailbox.
So far so good… I can see their folders and email.
However I can I sync their calendar from their IMAP mailbox to the suiteCRM calendar?
No, that is not possible out-of-the-box.
You can do some integration the other way around, SuiteCRM can publish an URL with an iCal so that external programs can show the SuiteCRM Calendar info.
You can search the SuiteCRM Store for add-ons doing these integrations, you will find some there (but you will need to pay)