Thanks so much for the response Diederik! Sorry it wasn't clear but what you explained above seems to be pretty close what I am hoping to get from Marketo.
Our issue is that there might be some instances where a lead is created and then instantly converted into a contact. We would love to still have that contact interact with (for example) a Marketo form on a webpage and have the data from the form go directly to the contact profile. My fear is that marketo will create a new lead profile since the lead was potentially converted. Is marketo smart enough to know that a contact exists in salesforce and that it shouldn't create a new lead? Maybe I am thinking about how marketo creates leads in a totally wrong way.