Here is a solution to the problem. Thanks to koniiiik i tried to solve this by extending the *get_fieldsets* method
class ProductAdmin(admin.ModelAdmin):
def get_fieldsets(self, request, obj=None):
fieldsets = super(ProductAdmin, self).get_fieldsets(request, obj)
fieldsets[0][1]['fields'] += ['foo']
return fieldsets
If you use multiple fieldsets be sure to add the to the right fieldset by using the appropriate index.