How do I convert a Workgroup (network) license to a Node Locked?

The safest method is to un-install and reinstall the product, selecting Node Locked license when you are reinstalling.

If you are comfortable changing your Windows Registry, and understand the risks you can simply change the following Registry key values:

On a 32 or 64 computer change this key to false:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths\TransMagic.exe\Network