Drop down - multiple choices within the fields

There isn't a default 'insert dropdown' option in Storyline that I know of, but you could accomplish what you're trying to do in a variety of ways. Mel's example above is a great way to do this using hotspots and triggers (his example coincidentally includes 2 buttons with 3 options each). You could also create a quiz (select Quizzing when creating a new slide and choose "Sequence Drop-Down"), but I think this isn't what you're looking for. If the final result is the learner performing work in a specific outside program (ie; Excel), then my software simulation example above would be the best method, along with ensuring they are learning in the exact format they'll be working in. Here's a link to a great step-by-step tutorial to create screencasts and simulations.

One thought: you mention you intend to have hundreds of examples in your training. That could be an exhaustive process no matter how simple the tools you use. Are you trying to illustrate to the learner how to do the data entry task, or have them simulate a complete job? I would suggest using a small example of the skill/process they are learning rather than re-creating the scope of the total project in your training. It will be more engaging for the learner and far less time-consuming for you.