Can you open a score and add VDL instruments?
You could import a House Style that is based on the VDL Template, but I don't recommended this method.
Or do I need to rewrite the whole piece in the VDL Template?
You won't have to rewrite the whole piece (although that is an option). So...
Setup a copy of the VDL Template, placing the VDL instruments you want in it, and then add the pertinent wind instruments to mimic the other file. Then copy/paste the contents from the other file into the VDL based score. (This method will help to alleviate possible file corruption from unintended side effects of importing a House Style.)