Hi Milchy,
Besides James' solution, you can make use of system variable @ISE.
This variable is used to determine if the empty column/sheet/book can be used to import.
i.e.
script after each import
if (doc.dropindex==1) tempise=@ISE; //backup the original value
wks.ncols = wks.ncols + 5;
@ISE=1;
script after all files import
@ISE=tempise; //roll back as original
del -v tempise;