If they are accessing SQL data sources via ODBC, then it would be presumably possible to work with filemaker's ODBC drivers as well. However, that is custom development time they might not be willing to put in specifically for your application.
Remember you need filemaker server advanced to act as an ODBC data source for outside applications.
You can read up on it here:
There are many other excellent commercially made calendar/planning products that are specifically designed for FMP - you might want to look at www.seedcode.com and see if their calendars might do the trick: I've used their free calendar add-on for my (very simple) solutions and it works great.