join-pane is the answer. I too was having problems with my attempts to use it based on the tmux documentation. I discovered that the -t and -s switches seem to accept [session]:window and not [session:]window. That is to say that specifying the session is optional, but including the : is mandatory. (I am using tmux 1.5)
Therefore, in order to add a pane to the current window and place window 1 into the pane, the command would be (Ctrl-B or whatever your bind key is, followed by)...
:join-pane -s :1
You can then break them appart with break-pane which by default is: Ctrl-B ! If you want to bind it to a shortcut, I suggest NOT overriding a default binding like s, because down the road you will look on the internet for an answer that involves choose-session and it will not work on your system. Notice that break-pane is bound to ! and @ is right next to it and not bound to anything by default. For that reason I suggest this binding...
bind-key @ command-prompt -p "create pane from:" "join-pane -s ':%%'"
The most important thing is that your LEARN "join-pane -s :" because if you just set it and forget it, you will be gimped when you find yourself on a foreign server. And let's face it, the most important thing a terminal multiplexer gives you is reliable sessions on remote servers.
This completes my conversion from GNU Screen to Tmux. I'll never look back.