Hi!
The value can only exists one time at the import process. Thats a big Problem for you.
You have to set the tags manually after importing the "categories". 
The import wizard is "at the beginning".
Another thing:
I have found the solution to run the import wizard again if the import fails and the attributes are marked in the wizard as "already imported":
You have to uninstall the vSphere WebClient Server and install it again.
The data isn´t stored in the VCDB, it´s locally on the WebClient Server.
I have opened a SR at VMware, I´ll keep you up to date...
hear us, Johannes!