Hi zivski
After importing to com_content, a K2 extra field group will be converted to an associated content type in com_content, an associated XML file will be generated:
/plugins/system/jacontenttype/models/types/your_migrated_k2_extra_field_group_name.xml
You can edit this file and change the field type as you wish.
About the frontend display, could you share the URL of page and screenshot highlighting the place you want to show?