Append Text to Description Field

Hello,

I have a script that I am using to take a list of computer objects, update the description, disable the ADaccount, and move the adobject to another OU.  This script works perfectly in my lab.  I would like to be able to append to the current description, rather than overwriting it.  Here is what I am using for the Set-ADaccount line:

Set-ADComputer $ADComputer -Description "$($_.Description) - Computer Disabled on $(Get-Date)" -Enabled $false

Unfortunately, this still just overwrites the Description field.  What should I do differently in order to append to the field?  For example:  "Sharepoint Server - Computer Disabled on 7/16/2014"

Thank you in advance!

Regards,
BrianSW