This is a bit more complicated -- you need to add a new entry to weapon.txt, the soldier.txt, and the RUTeams.txt. You will definitely want a tool (like QClone) that shows you the meaning of the various data columns while doing this. I would definitely create a backup of your original weapons.txt / soldiers.txt / RUTeams.txt, so you can go back to them easily if you make a mistake.
First you need to copy the whole row for the AT rifle you want to modify in weapons.txt and add it to the end of the file, updating the weapon class for the new entry to the next highest value. Edit this weapon so the target types are as described above.
Then you need to create a copy of the AT rifle soldier in soldiers.txt, add that to the end of the file, and update the soldier class to the next highest value. Then update the soldier's weapon so that the weapon class that used to indicate the original AT rifle now has the value of the new entry you added to weapon.txt.
Finally you need to create a copy of the AT rifle team in RUTeams.txt, add this to the end of the file, update the team class so it is the next available value in the file, and then edit the soldier list for the team so that it indicates the soldier class of the new soldier you just added to soldier.txt, rather than the old value of the original AT rifle soldier.