Can adaptive (conditional) questions in a questionnaire be created only via the API, if at all, in Captivate 9?

What API are you referring to?

Questions can be created externally, and imported, using the Moodle question format: GIFT.

The standard quiz slides don't really lend themselves to the kind of dynamic quizzing that you are talking about here, but it is possible using  variables and Conditional Actions to selectively navigate to different quiz questions according to the answers given by the user in previous questions.  But you probably need to enable backward movement in the quiz (because it might be hard to predict the order of the questions) and you would also probably need to be using Branch Aware (because not all question slides will likely be visited by the learner).

You could do this kind of thing more easily with custom built questions on normal slides, as that would allow you to use variables and Conditional Actions to change the text in captions etc.

However, this kind of complexity is not something normally recommended for a newbie Captivate user.  You would need to be quite familiar with the tool and have a very good idea of what it is you are trying to achieve.