Is there any reason you cant export the list in CSV format from your list of bounced emails, add them to a List in Marketo, and then make sure to specifically exclude those contacts from any Email programs or campaigns sending out emails? Alternatively, you could also import that List into Marketo and run a Data Value Change program to change the Unsubscribed field to true?
As Hobie stated, importing a csv might be your best bet. Upload your static list with 'email address' column and 'unsubscribe' column where unsubscribe equals true.
You can import them straight away in Marketo, in a dedicated list, and flag them as unsubscribed after the import.
You can also import them and include the "Unsubscribed" field in the import file, value = 1
Last point, read this, it might help you: Feeding the Durable Unsubscribe List without increasing the database
I would do exactly as Greg said, upload the leads with the Unsubscribe value, at least then you have them in Marketo to track activities on your website etc if and when they fill out forms etc.