check out the /filter command.

/filter -wfc @window filename.dat *
sends the entire @window to disk, and can be restored the next time with
/window -h @window | /filter -fwc filename.dat @window

If you're just needing to store the data, you'll be advised to try using hash tables, as mentioned above. https://en.wikichip.org/wiki/mirc/hash_tables
They're much faster than a @window because they don't need to waste time displaying the text too, thought that can be minimized by -h hiding your @window.