You can already control whether withdrawals are enabled or disabled for a group of users (Role), using the capabilities menu.
As for your other requirement, there is no need to use a blockchain address. Simply do an internal transfer (move). You could override the
[wallets_move] UI to provide a UI that always sends to the admin account, using a hidden input field rather than a visible user field. Please see the documentation on how to override the knockout UIs. If you create your view in
wp-content/plugins/wallets/includes/views/move/toadmin.php, then you can use a shortcode such as