You can copy any meeting and use it as a template for your own meeting. Tap "More" at the top of a meeting and then "Use meeting as template".

Please note that you are not extending a meeting, but creating a separate meeting. Meetings are usually independent of each other unless the other group knows about it. Therefore, adjust your meeting accordingly so that there is no danger of confusion. Choose your own title and change the description of the meeting. You can edit your own meeting at any time by tapping on "More" - "Edit meeting" at the top of the meeting.

You can also copy and reuse your own meetings :)