Let's say you want to add the column C where C=A+B?
you can do cat *.tsv | ./coladd.py -a C=A+B
if you do add more fields, separate your equations with ',' like this:
cat *.tsv | ./coladd.py -a C=A+B, D=(A/C)
eval() and csv.DictReader have been leveraged to achieve this task.