I don't believe Digi has published about the FIM peripherals beyond what you see in the Rabbit 6000 documentation (https://www.digi.com/resources/documentation/digidocs/pdfs/90001108.pdf)
As such, I don't think they're going to offer much support or additional information about the feature beyond what you can find on the website. I don't personally know why the feature wasn't promoted more (for example, by providing pre-compiled FIM code or documentation on setting up a development environment and writing a "Hello, World" app for a FIM).
I would advise implementing your CAN bus code within the main program on the Rabbit 6000. It will be easier to develop, debug and maintain in the long run.