It is possible to use your existing Microsoft Teams Room System (MTR) for joining external WebEx or Zoom meetings.
The requirement is to make sure your MTR devices are running the Microsoft Teams Room App version 18.104.22.168 or later. The update has been out for a while, since it was released back in July 2020.
For the configuration steps, I will not dive into this. Since Graham Walsh has described it in good detail in hos blog post: Microsoft Teams Room Guest Join Access – Webex
The process that Graham has describes is the configuration steps for WebEx, but the steps are the same if you want it to support for Zoom meetings as well.
After this configuration has been done. Then all your users will have to do, when they receive a WebEx or Zoom meeting invite and want to take the meeting in the meeting room with the Teams Room System. Is to forward the meeting invite to the MTR meeting room and the Zoom or WebEx meeting will be available on the MTR device.
Tom also had a blog post describing how Zoom and WebEx meetings work with Team Room System: Join Cisco Webex meetings from Microsoft Teams Rooms with direct guest join