Unfortunately, no. 36 MP is the total number of pixels in a full sensor exposure on a D800. If you crop, whether in post or in-camera via DX mode, you're deleting pixels. The pixel density of the D800 is actually slightly less than the D7000 (even in FX mode), and significantly less than the 24 MP DX D7100, but this doesn't really speak to IQ. Pixel density is nothing more than number of pixels divided by physical size of the sensor. If it were possible to measure IQ based on number of pixels or pixel density, then the D800 would be 2 to 3 times better than the D4, which it clearly is not!
But back to the point, the main difference in cropping a full frame exposure (assuming you used an FX lens) is that you can choose exactly how much to crop whereas in DX mode you're maxed out at 16 MP, which gives you an image that is a close equivalent, as it's been reported by many, to that of a D7000 in both size and various measures of image quality (and also very similar pixel densities). I have a D7100 and love it and also a D800. I seem to be in rarefied air when I say I'm thrilled with the DX mode (and 1.2 crop mode). 16 MP is PLENTY! And if you get the MB-D12 grip and put in either the D4 battery or AA's you can get 6 FPS in cropped modes. You can also try the bracketing trick from here http://www.dpreview.com/forums/post/51004727
to get 6 FPS without a grip! The difference between 4 and 6-7 FPS is huge and makes up for some of the ground this camera gives up to the D4. It doesn't turn it into a D4, of course, but it makes the D800 even more versatile than most people give it credit for, especially if you use the right glass. And this is the right place to find out what that glass should be!