bundle exec rails g migration AddExtraContentToProducts
class AddExtraContentToProducts < ActiveRecord::Migration
def change
add_column :spree_products, :extra_content, :text
end
end
then run the migration
bundle exec rake db:migrate
Read more about deface here
app/overrides/spree/admin/products/_form/add_content_field.html.erb.deface
<!-- insert_bottom "[data-hook='admin_product_form_additional_fields']" -->
<%= f.field_container :extra_content do %>
<%= f.label :extra_content, Spree.t(:extra_content) %>
<%= f.text_area :extra_content, :class => 'fullwidth' %>
<% end %>