You can get the rooms that a channel has, if any, by using the karken/chat/<channel_id>/rooms endpoint. From there, you can send a JOIN command using the channel_id and uuid like this:
Send("JOIN #chatrooms:" + user_id + ":" + uuid);
The same concept applies to leaving a room or sending a message in a room.
Send("PART #chatrooms:" + user_id + ":" + uuid);
Send("PRIVMSG #chatrooms:" + user_id + ":" + uuid + " :" + trailing);
A general note: the channel_id and user_id are the same thing in the Kraken API.