In the past I can find Panel available in Interface Builder's Library, but in Xcode 8, I searched the whole library in storyboard editor, no Panel available. Has Apple just removed NSPanel or I got something wrong?
Assuming you want to add a second window (as a panel) to your storyboard, add a new window controller. Then select its window and change its class to NSPanel. In the attributes inspector you will find that your panel can then be changed to regular, utility or HUD.
For Cocoa apps, I find that Nibs/Xibs are sometimes easier to work with than storyboards.