Hey Jamie,
There may be a work around on the SFDC side, where you add them to a SFDC campaign and when that happens they have an opp created... but I am unfamiliar with that aspect of SFDC. From the Mkto side, there is no way to create an opportunity. In that Mkto deals with Leads (and Contacts), Mkto only has visibility into Accounts and Opps, but they cannot modify them.