you would use /clipboard to modify the contents of the clipboard.
if you have a dialog list, then you can use //clipboard $didtok(dname, ID, 10) to fill the clipboard with each item separated by $lf, which mIRC will recognize as separate lines in $cb(), for example.
in general, you would use /clipboard -an <line> to add individual lines to the clipboard which you may prefer to do, in a loop, depending on the nature of the list.