No, its not possible to modify LFS in the way you specify without completely hacking it to pieces and changing the formula used for calculating the downforce. I suppose, in theory it would be possible to modify the car's "setup" details in memory, on the fly. But I've never specifically looked at doing this, and I have no idea whether it is possible.
I had the impression you wanted to plug the numbers into an excel spreadsheet, run some calcs and then dump out a .set file rather than that. To take that approach, it would be possible if you either;
* wrote a bit of vbscript, within the spreadsheet, to create the set file
* or wrote an excel plugin to do the same thing