Frustrating? Yes. Annoying that Apple leaves out such obvious shortcuts like this sometimes? Definitely, yes. Unfixable? No.
In the Keyboard & Mouse System Preference, you can create your own keyboard shortcuts for any menu item in all 'standard' Cocoa apps (meaning not Carbon apps like Adobe Photoshop and Microsoft Office or Java apps like... who uses Java apps). You can even do it to Firefox with their stupid XUL chrome.
I always use Command + Option + Shift + a letter/number for my custom shortcuts to avoid namespace collision.
It might seem like a mouthful but I use my left thumb for both Command and Option and my left pinkie for Shift, leaving three fingers on my left and my whole right hand to do one measly key.
You can scope your custom keyboard shortcuts to a single application (Numbers.app in this case) or make them global so that they work from any app. I used Command + Option + Shift + Down Arrow and Command + Option + Shift + Up Arrow for "Add Row Below" and "Add Row Above" respectively.
And you can do it, put your thumb into to it.