Just be honest with them. Tell them exactly what you're feeling and why you want to join one of those branches. There will be just as many opportunities when you come out of one of those branches if you get training in the right field. All I mean is that if you want to be an engineer, then maybe...
I would definitely go in as an officer and not Enlist. You can become an intelligence officer and that seems more like the logical progression to become a CIA agent.