Can I (host) welcome (off screen) a panelist who joins half way through a webinar?

If you have the "Waiting Room" active you can greet the speaker there.

Hi @JanBallenj 

Keep an eye out for an upcoming feature in Zoom Webinar called "Backstage". This will do exactly what you want to do. 

In the meantime you could run a separate Zoom Meeting in parallel to your Webinar using the "different meetings simultaneously" feature, greet your Panelist #2 there and have them join the Webinar when you are ready.

The "Host" of the parallel Meeting would need to be someone else, or yourself from another computer - but you could JOIN the parallel Meeting from the same client where you are Hosting your Webinar.

https://support.zoom.us/hc/en-us/articles/360001120743-Joining-different-meetings-simultaneously-on-desktop

DM me if you need some extra help with this.

Hope this helps otherwise.

Rupert