Yes, there are two ways to add KMZ files to Web AppBuilder for ArcGIS. Although the Add Data widget in Web AppBuilder for ArcGIS does not provide the option to add KMZ files to the map, the KMZ files can be added via ArcGIS Online or Google Earth. To do so, follow one of the options described below:
Add the KMZ file to ArcGIS Online
Note: KMZ files are the compressed, zipped versions of KML files.
Convert the KMZ file to a KML file in Google Earth
Note: For more detailed steps on how to add data to ArcGIS Online and Web AppBuilder for ArcGIS, refer to ArcGIS Online: Add items and Web AppBuilder for ArcGIS: Use the Add Data widget.