You unfortunately cannot import a 3D CAD line.
However, you can click the "Reset view" button in the top left corner, giving you a perfect bird's eye view. After that just click and hold the scroll wheel, and move around. This way you will stay at the same height at all times.
You can also try using the "Free flight" mode (fourth option to the right from the "Reset view" button), which mimics the drone's flight. You use the A, S, D, and W keys to move around, always staying at the same height.