I installed Lubuntu Linux in VirtualBox.

Is it possible to join a system directory with an external shared directory in VirtualBox.

I installed Lubuntu Linux in a virtual hard disk with smaller capacity by using VirtualBox.

When I installed Linux I installed some program packages from the repository that I need by using this command line for example
[code]
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install firefox
[code]

By default Linux always downloads the packages to be installed into the system folder
Code:
/var/cache/apt/archives
To save my virtual disk space I copied all the content from the
Code:
/var/cache/apt/archives
into my external shared directory on physical hard disk
Code:
/media/share/Linux/Packages/
Then I called this command to clean up the packages from the virtual disk space
Code:
sudo apt-get clean
Because I am still new to Linux I was in the situation to format my virtual disk and reinstall Linux again.

After fresh Linux installation I first copy all the content from the
Code:
/media/share/Linux/Packages/
back into
Code:
/var/cache/apt/archives
again and I start installing the packages I need so Linux does not have to download the packeges again from the Internet.

Is it possible to join Linux system directory
Code:
/var/cache/apt/archives
with this shared one
Code:
/media/share/Linux/Packages/
This shared folder I would like to make as Read-Only in VirtualBox
Code:
/media/share/Linux/Packages/
so I ensure the content not to be accidentaly deleted by Linux if I enter
Code:
sudo apt-get clean
casualy.

By this way there will be more disk space on virtual hard disk available to install more programs.

May somebody give the example how to join this two directories using 'mhddfs' command

Code:
/var/cache/apt/archives
and
Code:
/media/share/Linux/Packages/