It’s now working with Zoom: Virtual Webcams enabled in Zoom 5.0.4
In the meantime, WebEx is recommending that users join their conferences using the “Join from browser” feature when they would like to use Virtual Webcams in a video conference.
You can find information on how to do this here: https://help.webex.com/en-us/nrbgeodb/Join-a-Webex-Meeting#id_135013