It is possible create an assignment and send grades to an item gradebook manually created. But if someone change the gradebook item's title the assignment will lost the reference to the item gradebook, will have a reference to an item gradebook which doesn't exist yet. Also, if a teacher grades an assignment which is associated to an item gradebook whose name has changed, the grade won't be sent to the gradebook.
Steps to reproduce it:
- Add gradebook item, with name Item1
- Add new assignment, associate it with Item1
- Edit the gradebook item Item1 and change the title to Item1xxx
- Edit again the assignment. You will see that the assignment is not associated with any gradebook item.