It would be interesting to know and discuss more around creating bulk opportunities in Salesforce using Marketo. is it possible? thoughts?
The native sync cannot write/update the Opportunity object, however, I suppose you can use a webhook compatible service that could do that for you. What's your primary use case for this? I don't see a lot of people needing to write/update opportunity object from within the Marketo, let alone trying to find out how to do it in bulk.