b,a,c,

32
34
25
13

b,a,c,32,34,25,13

sed -e :a -e '{N; s/n/,/g; ta}' numbers.csv >> letters.csv

b,a,c,
32,34,25,13

$ paste -sd, numbers.csv
32,34,25,13

$ paste -d '' letters.csv <(paste -sd, numbers.csv) > tmp && mv tmp letters.csv
$ cat letters.csv
b,a,c,32,34,25,13

$ sed -i -e "s/$/$(sed -e :a -e '{N; s/n/,/g; ta}' numbers.csv)/" letters.csv
$ cat letters.csv
b,a,c,32,34,25,13

cat letters.csv numbers.csv | tr 'n' ',' | sed 's/,$/n/'

$ awk 'NR==FNR{printf "%s",$0; next}
              {print $0}
           END{ORS="n"; print ""}' letters ORS=, numbers |
  sed '$s/,$//'    # to delete last ","

b,a,c,32,34,25,13

awk 'FNR==NR{t=$0;next}{s=s","$0}END{print t s}' letters.csv numbers.csv