I have the same issue.
VCenter Converter Standalone 6.0 does not allow to choose anything different from VMware Infrastructure as Destination System, when I try to virtualize a remote powerd on Ubuntu 14.04 machine.
In other words, I can find no way to choose VMware Workstation or othe VMware virtual machine instead.
On the second step (Destination System) of the Convert Machine wizard there is no drop down menu where I can select VMware Workstation or other VMware virtual machine. The only fixed choice is VMware Infrastructure.
(I tried to use the previous vCenter Convter version (5.5.3), but his does not even allow to go through the first step of the wizard (Source System) because it fails to contact the same Ubuntu machine...)
Ciao,
Carlo