asterisk users - Oct 2018 - Connecting an existing conference via PJSIP?

Let's say I have a conference room of 8 users. At some point in the
evening, we need to hook up with a Zoom conference.

That means hooking up that existing pool of users to a new PJSIP
channel. An admin would dial in, enter a pin, and initiate that
connection.

Sounds really simple, but I've read:

https://wiki.asterisk.org/wiki/display/AST/ConfBridge
https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Application_ConfBridge
https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Configuration_app_confbridge
https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Function_CONFBRIDGE
https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Function_CONFBRIDGE_INFO

Perhaps I'm not seeing the wood for the trees, but I'm not figuring it
out.

I think I need to set a variable when the admin dials the pin, and
then within that variable, write the name of the new PJSIP channel
which the admin caused to happen.

And then join the existing channel into the conference. That's the bit
I can't figure out.

Thanks.

Oh, and finally, when dialling the Zoom channel, I think I need to use
https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Function_PJSIP_DTMF_MODE
to "PJSIP_DTMF_MODE(inband)"
because in PJSIP wizard, I've set "endpoint/dtmf_mode=rfc4733"
otherwise it doesn't work with everything else.
And of course, Zoom needs audible touchtone digits.


(Before anyone says "Why not use Zoom Sip Connector" - well, £39 per
month for the SIP connector, or 3*10 minutes at 1p/minute (ie:30p) per
month via PSTN is why!