Hi Peri,
You just want to create a custom field and stick it on your form.
Go to Admin:
>>Field Management
Probably create a string field in this case as I'd imagine your'e going to change it to a picklist of preferred dates.
When you put it on the form change the field type to select and put in your options. That will be a "Marketo Only" field and will provide the data you need. Doesn't need to be synched to SFDC.
Cheers,
Michael