I wanted to share a video, but it wouldn't be good to put the link here, so the link could be censored.
No, you need to unify the empty disk to the Ubuntu's HHD. VMWare to Virtualbox can't detect the HHDs or the volumes. But if you have enough Ubuntu's HDD, don't unify, firstly install the virtual machine on Ubuntu's HDD, then, move it to the empty disk.
But search "OS X Snow Leopard" at Web UPD8 and you'll find that article. Google at Youtube "How to install Mac OS X Snow Leopard on Ubuntu".
I recommend Virtualbox as that article and the video suggested, so it's very easy to be installed. But VMWare is another good idea.
If you want to use OS X 10.9... firstly you need to install OS X Snow Leopard on a virtual machine and update to 10.6.8, and at 10.6.8, download OS X 10.10 (it's free) (unfortunately OS X 10.9 isn't available).
I recommend you to visit and ask for help at the InsanelyMac (they're good people and help a lot the people to make Hackintosh), but before, I'll give you a tip: please, don't cite iAthos and TonyMacOSX86, so that forum doesn't support them.