How to create a vmdk file from product.iso?

I've just started with turnkey linux and tkldev, in order to create an easy-to-use environment for the participants of a drupal workshop.

I could make a core and drupal7 product.iso, but I could not figure out how to create a vmdk file from there.

Sorry for putting such a simple question, but I could find only a thread for the other direction (vmdk to iso).

Any help appreciated - cheers, Joachim