Hi Syed -
You can create two custom fields to achieve this.
I would reccomend you create the fields in SFDC first, and let those fields sync over to Marketo.
Opt-in would be a checkbox, with the default value left unchecked.
Opt-in would be a date field. You would change data value based on a trigger of data value changes for Opt-in, new value = True.
Let me know if you have any questions.