To add to Cathal's suggestion: if you create Marketo-only fields, you could create a Smart Campaign that copies the values in those fields to the fields that are synced with SFDC, but only under certain conditions. You can create the rules in your Smart Campaign. You would use a Change Data Value flow step, e.g.:
Change Data Value, attribute = "Field XYZ in SFDC", new value ="{{lead.marketo only field}}"
Hope that helps,
Jep