Wow, good question.
I'd speak with your SFDC Admin and Support to help with this migration.
I suspect you will need to recreate these in SFDC and do some db magic to get them to sync. I assume you meant you wanted them to sync to SFDC...otherwise any custom fields that are Marketo only can live happily w/o a sync.
Right - sorry, the intention is to show up in Salesforce and NOT have the integration re-create them back in Marketo as I think will typically happen. I was hoping there is a simple solution. I'll call up Marketo support and see if they have any ideas - maybe they can merge fields or something. Thanks Josh.