You cannot modify server side code on the hosted solution. However, you can add custom computed fields to a model which can be based on python code that is entered through the UI.

For example, if you want the Invoice Total in words, add a computed field called  x_amount_total_in_words to the account.move model like so. You can then use the computed field in QWeb like a normal field.

Python Code:

for record in self:
    record['x_amount_total_in_words'] = record.currency_id.amount_to_text(record.amount_total)