There does not seem to be any documentation on how to do it. I went into /sdk/installer and ran the vboxsetup.py script but it raised the following exception:
Exception: No VBOX_INSTALL_PATH defined, exiting
It is really annoying that they don't adequately explain how to install it and it's weird that the virtualbox installation does not create the VBOX_INSTALL_PATH variable by itself.
Anyways, This is what worked for me:
export VBOX_INSTALL_PATH=/usr/lib/virtualbox
sudo -E python vboxapisetup.py install