requesting an additional disk for a VM

Hi All

What is the best way to allow a user to be able to request an additional disk to be added to an existing VM in vCAC?  I'm assuming its a vCO / Advanced service thing but I can't see a way of doing it.

I'm not so bothered about having the OS automatically configuring the disk but if there is some guidance on the best way to approach that too it would be helpful.

Thanks

Danny