I might not understand what you mean by "extend the provided use case", but remember what the instructions-file says:
A business analyst has worked with FBN to define the use cases and create a business domain model. You may not agree with the what the business analyst came up with, but the business analyst is no longer available to discuss the situation. This means you have all of the requirements you will get [...]
So, in other words, you don't quite agree with the business analyst. In real-life, outside of Sun Assignment World, disagreements between a BA and an architect are not uncommon.
Has the BA forgotten a use case? Is it an important use case? Do you need to elaborate the use case (using a sequence or collaboration diagram)?
You have all the requirements you will ever get. But if you see something missing, bring up the point. If a domain object is wrong or missing, mention it, and also state what action you take. You might find that this would require a new use case, if so - specify in a few words that use case. You could even add more sequence or collaboration diagrams, if you feel it adds value. It is up to you.
The required deliverables are: one class diagram, one component diagram, and one sequence or collaboration diagram for each use case. But this is not a hard requirement - I made two class diagrams, one component diagram, and seven collaboration diagrams, others have delivered I one class diagram, four or five component diagrams, four use case diagrams (one for each detailed use case), and also other combinations.
Do and deliver what you feel is required to describe solution, and don't deliver anything that does not add value.
Sorry for not clarifying that. My concern was if it is possible to add detailed use case diagrams aside from the one provided. And I guess your reply had already covered it. Thank you very much for sharing your experience and insight. I�ll keep in mind you advice.