Hi, there are two ways to do this, the most elegant and using a library for python called "pandas" this is the link for their site, you can install using the python pip and have all the documentation on their site.
But I'm going to write a less elegant way here, but it solves your problem.
Entry: data.csv
Serial; Imei; Número; Icc; Stock
869606020314526; 869606020314526; 934097609; 20000736862016; Stock
Script: numeroUpdate.py
file_in = open('data.csv','r')
file_out = open('numero_atualizado.csv','w+')
next(file_in) # Pois queremos pular a primeira linha.
file_out.write('Serial; Imei; Número; Icc; Stock\n') # Cabeçario do arquivo, e o "\n" é para pular de linha.
string = '' # Criado para podermos chamar o metodo join.
for line in file_in:
line = line.split('; ') # No split, cortamos string.
numero = "+244" + line[2]
line[2] = numero
file_out.write(string.join(line))
file_out.close()
I think it can help, anything, just comment.