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This question really belongs to Products . The specified item was not found. is really for generic marketing questions. You should move it, you would get more numerous and faster answers.
Once the custom object is created in Marketo (and in Marketo first), there is no possibility to send it back to SFDC. The only SFDC custom objects that can be sync'ed have to be created in SFDC first and mapped. And even though, the sync in one-way, from SFDC to Marketo.
If you really need this data in SFDC, you will need to go through some custom integrations (API or ETL). Not all ETL out there support Marketo custom object, though, so have a close look if you want to go that route.