You can control the apps that your users extend their Chromebooks with by enabling the use of Android Apps from the Google Play store.
This requires a couple of steps but it's easy to do. If you get stuck please reach out to our team for some support
Go to admin.google.com > Billing > Subscriptions
In the left menu select Devices and Android Management
Follow the process to enable this and add to your account. It's free.
From there go to Devices > Chrome > Apps & extensions > Users & Browsers
Click on Additional Settings and find the 'Android applications on Chrome devices. Change this to Allow apps on the Google Play store.
In the Allow/block area decide if you want to allow all apps (at your users discretion) or if you will maintain a list of the apps they are allowed to install (more secure, but more work for you).