#!/bin/bash
s='1,2
3,4
5,6'
awk -F, '{print $0 OFS $1+$2}' OFS=, <<< "$s"
echo "---"
awk 'BEGIN{FS=OFS=","} {print $0 OFS $1+$2}' <<< "$s"
echo "---"
awk -F, '$0=$0FS$1+$2' <<< "$s"