This is a terminology issue that comes up a lot and creates confusion.
1. Everyone is called a "lead" in Marketo, but in SFDC, "leads" equals pre-sales prospects and "contacts" are pre-sales prospects that have now made contact with sales and are either in the sales cycle or have become customers.
2. When your CRM contact is synced into Marketo, it is still a contact in your CRM. It won't change into a leads to go through the MQL convertion to contact again.
3. If a lead in Marketo is created and is not a contact in your CRM it will sync to your CRM as a lead (if you sync things that way) and then you can nurture to MQL and convert to contact.
4. In SFDC some orgs convert everyone to a contact, and if you want to watch the lead to contact conversion in that situation you can't to my knowledge. And so people will mass export those names, delete them from salesforce and reimport as new leads into Marketo. YMMV.