Hi,
I don't think that is possible using the AD Plugin for vCO (because this doesn't expose the functionality).
However, nothing is impossible in vCO, you just have to be more creative...
My next try would be: Use the Powershell plugin to call out to a powershell script that does the job. Quick google found some examples, I'm sure there are plenty more: http://dmitrysotnikov.wordpress.com/2007/07/25/set-any-ad-attribute-with-powershell/
http://stackoverflow.com/questions/6603519/modify-attributes-in-ad-via-powershell-no-quest
Cheers,
Joerg