Add existing .hdd or .vmdk doesn't work

I'm trying out Parallels as a potential replacement for Virtualbox, but having used Virtualbox for some time, I have a number of virtual hard disks that I share between virtual machines in Virtualbox. I would like to be able to add these to virtual machines in Parallels, but I can't get it to work.

When I try to add a .vmdk or .hdd to my virtual machine in Parallels, I get the message I've attached a screenshot of. If I click convert, some message briefly flashes on the screen and then the dialog closes. In other words, it doesn't convert the virtual hard disk.

Any idea what's going on?

Cheers,
Rickard