I'm wondering how easy it is to add additional fields to the MLM form for example gender & age, country etc so these can also be added to the database. I'm using the text file option so I don't need to create extra tables in a MySQL database or anything as complicated as that.

You just need to the new fields to your form and make sure they are named properly. Optional fields are opt1, opt2,... to opt20. You can look at the other fields to see how it is done.