Powershell Set extensiondata (String to Byte array) Active Directory

Hi,

could anybody help me to write the attribut "extensiondata" to AD?

I'm getting the data as follow:

Get-ADuser -id 'tjones' -Properties extensiondata | select -ExpandProperty extensiondata  | ForEach-Object {[System.Text.Encoding]::ASCII.GetString($_)}

The result is:
ZHMAILBOX
WEBMAILBOX

I like to write the string ZHMAILBOX to ByteArray and set extensiondata with this bytearray.

Thanks in advance