You sure can! On the landing page, how are they actually opting back in? Regardless, you can create a campaign that upon the form being filled out, change the Unsubscribed to False. Use the "Fills out Form" trigger and reference the form. In the flow, use the Change Data Value flow step. Enter Unsubscribed for the attribute and false for the new value. Then activate!
If they have the option to stay unsubscribed on in the form, then you can build this out based on their answer. Just use the Data Value Changes trigger; reference the field you are using to determine whether they want the emails or not, then add the New Value constraint and reference the yes answer. The flow will be the same.
Note: Be careful with emailing unsubscribes. Could be a slippery slope with CAN-SPAM laws.