Unfortunately, the UberConference app is not available for Chromebox for Meetings. Chromebox for Meetings currently does not support Hangouts Apps.
A workaround is to have the Chromebox for Meeting dial-out to your UberConference number. This way, you'll have a bridge connecting both systems.
This is not optimal since you won't be able to see the UberConference interface with who's on the call, who's talking, or any of the call controls.