New field for existing custom object

I have a new field coming to MKTO from SFDC called "Form Product Type". 

I'm hoping to put this field into a Custom Object that we populate via API Forms from our Drupal CMS system. The field is going to be a hidden field that populates the field value based on the product/page the person submitted from, so if they submit a Form from the product A page, that field would populate with the "Form Product Type = A". And so on for product pages B, C, n.

The problem I'm running into is that the Object already exists and I can't seem to add a new field to the object.

What can I do?