Enable Dev channel

Type the following into chrome browser address bar:
chrome://flags/#enable-experimental-crostini-ui

reboot

Go into chrome settings, look for linux (Beta) below "Play store" in settings, enable it. 
Click install
Reboot

The following website talks about installing various windowing systems in crostini
http://linuxiumcomau.blogspot.com/2018/07/introduction-to-crostini-part-1-hp.html


Follow this procedure to add virtualbox to your setup on crostini

• apt install virtualbox

Oracle Virtualbox third-party repository

1. Add virtualbox.list to /etc/apt/sources.list.d

   deb http://download.virtualbox.org/virtualbox/debian stretch contrib

2. Add Oracle VirtualBox public key:

   wget https://www.virtualbox.org/download/oracle_vbox_2016.asc
   sudo apt-key add oracle_vbox_2016.asc

3. Install virtualbox-5.2

   sudo apt-get update
   sudo apt-get install virtualbox-5.2

Complete instructions on the VirtualBox Wiki

VirtualBox can now be started. To not load the VirtualBox modules at system startup, edit /etc/default/virtualbox and set LOAD_VBOXDRV_MODULE to 0.