I can see 2 ways to implement it.
1. to redirect to a page with the terms, click on "I agree" and then proceed with the registration, or
My forum deals, among others, with financial and investments topics and I have to protect my interest and not be liable for false advice and/or recommendation of my users.
"640K ought to be enough for anybody." - Bill Gates, 1981
I'm sorry but this is not planned in near future, but it's already in our to-do list.
Currently you can customize wpforo/wpf-themes/classic/register.php
Theme customization quick guide: