Yes. FreeBusy lets you link as many calendars as you have, irrespective of which calendar system each one uses.
Step 1: Go to FreeBusy Calendars and Integrations
Step 2: Click on Link another Calendar
For example, you could link your Salesforce, Google, Zimbra and Exchange calendars all together. When you share your availability with others, FreeBusy will present a combined availability view from all these calendars.