Notepad++ has a feature that may work if you can make the replacement with a [regexp] expression as a substitution.
Alternatively, there are editors with batch-capable commands that can operate in batch file mode in a for...each mode on a file list.
Or, you can write a Matlab script that does the job -- if the line goes in a fixed location like at the end, it's pretty trivial if the files are either named with a well-defined name that is usable wildcard pattern or are all in a given location --
Alert -- air code, test thoroughly before destroying anything by having backups!!!
nL=100; % target line number in file
L="Your new text"; % the text to add/insert
d=dir(fullfile('yourfolder','yourfile*wildcardname.ext')); % get the list of files
for i=1:numel(d)
fn=fullfile(d(i).folder,d(i).name); % the fully-qualifie fn
F=readlines(fn); % in memory as string array
F=[F(1:nL);L;F(nL+1:end)]; % insert the line where wanted
writematrix(F,fn) % rewrite modified file
end
The above overwrites the original file, obviously you can modify the filename before writing to avoid that...
