Hello Lael!
K2 is a component that uses a different storage (table) than Joomla. SEBLOD on the other hand uses Joomla tables and enhances the content.
What you could try to do is create content type which is free (Object: Free in Form&Content type), and create fields that are connected to columns in your #__k2_items table (storage). I haven't tried this, so I do not guarantee success, but it is there and I am quite confident it will work.
However, be prepared for different table structure than Joomla, even more so regarding custom fields. which might be stored as JSON in extra_fields or in a #__k2_extra_fields (I suppose it stores field definitions, haven't used it in ages).