open your secure data with /secure and press alt v

type the following commands in your /secure buffer and watch.

to start with set autojoin to your soon to be created /secure variable

/set irc.server.freenode.autojoin ${sec.data.joinlist}

create your secure variable with a few channels

/secure set  joinlist #chan1,#chan2

add further channels to your secure variable

/eval /secure set  joinlist ${sec.data.joinlist},#chan3,#chan4