BuddyPress.org

Yes, you can!

If you have a child theme, move /plugins/buddypress/bp-themes/bp-default/registration/register.php and /plugins/buddypress/bp-themes/bp-default/registration/activate.php to your child theme and modify to your liking.

If you don’t have a child theme, read this guide:

https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/