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You will probably want to recreate the navigation bar. I do not think you can import it.

Here's what I would do. Load the page with the navigation bar, and mouse over one of the buttons so you can see the mouseover as well as the mouse down state, press Print Screen, then page the screen capture into Designer.

Create a button for the MouseOff state, copy the button, select the MouseOver layer in the Page & Layer Gallery and Edit Paste in Place.

Modify the MouseOver layer button to match the one in your screen capture.

For the button you need a shape, usually a rectangle, grouped with some text (Centered).

Let us know if you need more steps.