I'm going to guess what you're asking since I didn't find your explaination too clear.
You want to read in the file, and you want to append to a string exactly, including the whitespace that is consumed by the istream::operator>>(). This is quite simple, unfortunately, using the operator>>, you give yourself a problem. That operator will stop on ANY whitespace. Which means you'd never know exactly what it's stopping on unless ofcourse you used istream::peek(). Better yet, you should probably read in a whole line with the getline() function. This way you can declare a delimeter to stop on, and append it after each encounter.
while (!CInput) // Better not to use eof()
getline(CInput, FileLine, '\n');
FileData += FileLine + '\n'; // The line plus the consumed delimeter