WiFi hotspot without internet provider knowing?
Any unencrypted HTTP packets (web sites or even streaming commercial content like music) would by default contain the User Agent corresponding to the device and browser/app, which would make it easy to distinguish between devices. It would be rather involved to change the user agent for non-browser traffic; some of those packets are generated deep within the OS.

Effectively masking a WiFi-connected device would be very difficult, if not practically impossible (depending on the sophistication of IT), if it is iOS or tvOS, or especially if it is a non-Apple device. Different devices and OSes have different characteristics for traffic they create.

You could encrypt the traffic with a VPN on the WiFi-connected device(s), but this could easily be noticed and is likely to be a red flag to IT.

The easiest combination to hide would be a device identical to the wired device, e.g., a macOS laptop over WiFi to a macOS laptop. Even then, source port number patterns and other markers would be a good clue to IT if they are paying attention.

In addition, the mere act of connecting another device is problematic. Operating a WiFi hotspot is likely to be noticed, even (again, depending on IT sophistication) if you hide the SSID. Triangulation could probably isolate the location to within a few machines. You could tether a device using USB, Bluetooth, or another hardware port, but any connected device will also leave forensic clues on the wired device.