create signal group for existing matrix room

[aiomaster]

In previous versions of the mautrix signal bridge there was a command to create a new signal group for an existing matrix room.
It was called !signal create but it doesn't exist anymore in the help.
Maybe it is gone due to some refactorings.

I tried to get it working otherwise and found

signal/ROADMAP.md

Line 73 in 4b63486

* [x] Private chat/group creation by inviting Matrix puppet of Signal user to new room

and thought it would work by inviting the puppet to a matrix room (after inviting the signal bridge bot and the relaybot), but it doesn't work that way.
Then I read, that I have to manually update the mxid in the portals table: https://github.com/mautrix/docs/blob/ba35623221db24e86071bc73f2b9ef54607b7879/bridges/general/relay-mode.md?plain=1#L24
What exactly do I have to do?
Should I create another room in signal, bridge it to matrix and than try to change its mxid to the existing room?

My use case is to bridge a matrix room via relaybots to telegram and signal.
I have some rooms working that way, but I made it working with the !signal create command in the past, that is missing now.

Is it planned to reimplement it?