When you say raspbian perhaps you mean the lxde desktop- command to install that:
sudo apt-get install lxde
Then once you get it installed you need to go into sudo raspi-config and change the boot sequence to boot into a terminal- that way on boot it will still go straight into emulationstation. Then when you want to use the desktop you press F4 to get into the terminal from emulationstation and type startx.

see this video for a step by step: