Hi Colleen:
Here is how I would approach this:
1. Create a custom "How did you hear about us" field in Marketo (custom fields created in Marketo will not sync to SFDC).
2. Add the field to your form
3. Create a campaign that triggers off of the form fill-out.
In the flow, use a Change Data Value flow step with the following Choice: If Lead Source is empty, then enter the following token for the new value: {{lead.How did you hear about us}}. The Default Choice of the flow step would then be "Do nothing".
Add another Change Data Value flow step that sets the field "How did you hear about us" to "NULL" (Note: This must be all upper case "NULL" to reset the field - "null", or "Null" will not work)