Im using the BT Graphite 2500 for my small business, and wondering if its possible to add a custom hold message/song when pressing R.
That depends on who provides your phone service, and what "hold" sound options are available.
If its BT Business, then you may get an answer on the BT Business forum.
If you have your own internal phone system, then you would need to speak to the system manufacturers.
This is the BT Residential customer forum.