Vsphere-clone adding additional disks

I have been using packer for a while but mainly for aws ami creations. I am looking to port this over to using the vsphere-iso and vsphere-clone.

The practice I have followed with AWS ami’s is to create a base ami and create additional layers. One thing I normally add it layers is additional disks.

However I cannot see how you can do this with vsphere-clone ?

For example:
at the moment I have used vpshere-iso to create a base OS vm template, now I want to use this to create a new vm template with an additional disk and some installed software.

any help would be appreciated