MW can process fbl emails perfectly. If you have a mailbox destined to receive fbl complaints/emails, you can add that to MW and MW will do the processing automatically. This is the case when you're managing fbl on your own.
But in your case, you're using a 3rd party service who does this job for you. So, you don't have to sign up at various ISPs and process the fbl emails separately at your end, instead, Sparkpost will do that part. You're paying Sparkpost for taking care of that job as well. In MW, you can add/connect to your Sparkpost account/server using their webapi. If you use webapi, then you should be receiving bounces, complaints/fbl unsubscribes etc at MW from Sparkpost automatically and proper action will be taken.