Ben,
What you can do is create a similarly named field in Salesforce (e.g. Form Comments), wait for it to sync, and then use a Change Data Value to map the contents over.
Something like:
Smart List: (Filter) Website/Form Comments is not empty
Change Data Value: Field: Form Comments, New Value: {{lead.Website/Form Comments}}
Run Once
Then start using the Salesforce-synced field instead. Or if you have to keep using the old one, create a trigger campaign like the one above to map the data values from the Marketo field to the SFDC/MKTO joint field.