BuddyPress.org

It’s not straightforward, but it is possible. You’ll have to replace bp_core_screen_signup() (where the value of $bp->signup->step is set) with your own custom function. You’ll need to do the following:
`remove_action( ‘wp’, ‘bp_core_screen_signup’, 3 );`
to remove BP’s registration function, followed by
`remove_action( ‘wp’, ‘my_bp_core_screen_signup’, 3 );
function my_bp_core_screen_signup() { // etc`

Copy the contents of bp_core_screen_signup() into your custom function. Then start looking for the places where $bp->signup->step is set – that’s what you’ll need to mess with to modify the way that registration steps work.