Php member registration (manual assign Account-ID)

We have a php membership registration system which automatic generate the Account-ID while member signup. We would like adding one feature to the system : manually assign Account-ID while add member on admin panel. Service provider request build the following : Client side : 1. Search account-ID available 2. Form mail for registration information - Reply mail include Admin side : - Add one more add-account forms according (duplicate) to the existing add-account form, which modify the Account-ID field from auto-generation (not editable) to manual input (editable). All other should keep same as existing. - Submit to update database All modification must not affect any of existing features & operations, or any other things to existing system. We can provide test-server environments for developing need. After all testing is passed on test-server, service provider should support the buyer to complete and success implements the modules onto the live server with details instructions. Payment will place on escrow account.