The wireless add-on is available for pre0order at 7am PT Friday (November 11th, 2016) through Vive's Chinese website. Quantities are said to be limited and priority will be given to existing Vive owners with a valid serial number. It is being priced at 1,499RMB (~$220USD) plus shipping.
The wireless add-on on includes a built in battery which is said to be good for around one and a half hours of game play. They also have plans to have a larger battery which the user will put in their pocket instead.
It will be interesting to see if HTC will decide to integrate this technology into their next generation Vive VR headset.