View source for installing virtualbox extensions on linux HOST (NOT guest!)

==software versions==
O.S. = debian 10.0 (buster)
virtualbox = virtualbox-6.0                        6.0.8-130520~Ubuntu~bionic      amd64        Oracle VM VirtualBox


list installed packs
 $ VBoxManage list extpacks
 Extension Packs: 0




== from the manual==
Oracle VM VirtualBox comes in many different packages, and installation depends on your host OS. If you have installed software before, installation should be straightforward. On each host platform, Oracle VM VirtualBox uses the installation method that is most common and easy to use. If you run into trouble or have special requirements, see Chapter 2, Installation Details for details about the various installation methods.

Oracle VM VirtualBox is split into the following components:

    Base package. The base package consists of all open source components and is licensed under the GNU General Public License V2.

    Extension packs. Additional extension packs can be downloaded which extend the functionality of the Oracle VM VirtualBox base package. Currently, Oracle provides a single extension pack, available from: http://www.virtualbox.org. The extension pack provides the following added functionality:

        The virtual USB 2.0 (EHCI) device. See Section 3.11.1, “USB Settings”.

        The virtual USB 3.0 (xHCI) device. See Section 3.11.1, “USB Settings”.

        VirtualBox Remote Desktop Protocol (VRDP) support. See Section 7.1, “Remote Display (VRDP Support)”.

        Host webcam passthrough. See Section 9.6, “Webcam Passthrough”.

        Intel PXE boot ROM.

        Experimental support for PCI passthrough on Linux hosts. See Section 9.5, “PCI Passthrough”.

        Disk image encryption with AES algorithm. See Section 9.30, “Encryption of Disk Images”. 

    Oracle VM VirtualBox extension packages have a .vbox-extpack file name extension. To install an extension, simply double-click on the package file and a Network Operations Manager window is shown to guide you through the required steps.

    To view the extension packs that are currently installed, start the VirtualBox Manager, as shown in Section 1.7, “Starting Oracle VM VirtualBox”. From the File menu, select Preferences. In the window that displays, go to the Extensions category. This shows you the extensions which are currently installed, and enables you to remove a package or add a new package.

    Alternatively, you can use the VBoxManage command line. See Section 8.43, “VBoxManage extpack”.