Dave,
The SN file is in astorb.dat format, other than the object numbers being changed, and there being one less blank at the start of each line.
Why they changed the object number is beyond me. It makes no sense for the numbers to differ from the official ones.
Rather than trying to fudge the data from MPC or Horizons, I suggest you follow the instructions to get astorb.dat from Lowell:
http://www.naic.edu/~nolan/astorb.html
Once you have the base files, rather than downloading the whole thing to get updates, you can get the add/delete packets from the instructions further down the page. You only want the ones newer than the astorb.dat file you downloaded.
Then just cut/paste the ones you want into Asteroids.txt
You might have to change the object number to avoid duplicates, and if the comment about SNP only loading the first 100 (another strange idea - something to do with performance?) is correct, put your additions in the first 100 rows.
Otherwise I'd suggest CdC "Sky Charts"
http://www.ap-i.net/skychart/ version 3.1 patch level 1340 or newer which will load as much of MPCORB.DAT as you want.
Andrew