Lea, I did this with my dnn site. I am using version 7.0 or 7.2 I forget.... Ill try to walk you through it best I can.
1. Login to your site
2. Then navigate to Admin>Site Settings
3. Click on User Accounts Settings
4. Under Registration Settings, find Registration Form Type: and set it to Custom
5 Under Registration Form Type there is Registration Fields click on the fill text area
6. You can set the order here of how it appears when someone goes to your registration page Ex. If Username is the first entered then username will be the first field to show up.
7. If you just want to just add your custom field at the end, start to type it and it will auto search for it. Ex. if you have a field of Middle Name and start to type middle it should pop up Middle Name
8. Click on the field you would like to add and then the next and so on...
9. Don't forget to click update at the bottom of the page when you are done.
10. clear your browsers cash ect. and go to the registration page.

Hope this helps!!