Can't you just press Alt+L to open
mIRC Channels List dialog from there you can change the file it's saved as?
Here's something that may do the trick.
Raw 321:*: {
write -c $+(channels,$upper($network),.txt) $server $asctime
}
Raw 322:*: {
write $+(channels,$upper($network),.txt) $2-
}
If your on a network it writes to file channelsDALNET.txt for example. If you aren't on anetwork it writes it to channels.txt simply removing $network from the equation.

-Andy