Theoretically it should be possible, but not using FFP. A NSA3xx doesn't have LVM. (A NAS5xx does, but that's another forum), so you should use bcache. Using bcache you have to assemble a virtual filesystem containing your data partition and your SSD, and mount that. If you wait until FFP is started, the data partition is already mounted, and you can't unmount it because it's busy.
Using an usb_key_func.sh stick
you can patch some firmware scripts before the firmware assembles and mounts the raid array. You can intercept the mount command, and let it assemble the virtual filesystem and mount it, when the firmware tries to mount the raid array.
Maybe intercepting 'mount' won't work. It didn't work on an NSA320
, and you didn 't tell which box you have.