One way is with an Automated Action like this:

Code:

for record in records:
  if record.company_id > 1 and company_id.name[3] not in record.code:
    record['code'] = record.company_id.name[3] + record.code

Note: Don't put spaces in Account Codes - it prevents you from using the CODE to import.    Also, in this example, the prefix is NOT added for the first Company (because it was considered the parent Company)