Hi Fluxblocker,
You can basically do it correctly with the vCenter Converter. After you select the part of the Destination Location you will go to the Options now, in there you can basically select which disk you can only migrate together with your virtual machine on the basic settings or you can also go for the Advance settings where you can actually select the capacity of the disk you want only to migrate.
Hope I was able to help.
Cheers.