This module allows sending a message to another MUC room.
@other-room: hello
The message will appear in the other room as as
<first-room/You> hello
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_muc_intercom
For earlier versions see the documentation for installing 3rd party modules