Well, I did something similar in the past. The company gave me some standards that the new files have to meet, and they gave files as example input files. The technical lead was so happy with my performance and the end of the project.
I don't know how large could be the data that need to be transformed, I put 10 hours per week because I worked in a company at this moment, too. So I'm planning to take some hours in the night and in the weekend to complete the tasks. I could make some arrangements to spend more time if it's necessary. Also, I'm expecting to spend more than 10 hours if that is possible to me during the week, to deliver your product more quickly.
Let me know if I meet abilities that you are looking for and let's talk how we can manage the negative points to make this work.
Thanks.