I do not know the details of adding custom fields to the front end user form for buddy press.
Adding ACF fields to forms created by other plugins usually involves using acf_form() inserted between the form tags of the other form with the form argument set to false.
If the other plugin allows adding custom fields then you could also hook into the save action used by that plugin and build an action filter that updates the ACF fields.
In any case, what you can do depends on the other plugin and what you can modify about the operation of the other plugin.
