On the employees' RDO tab you can enter an opening balance of RDO hours and the date this is applicable from. RDO's will then calculate after that time as per your Daily Rule.
These two fields are available to import via the Setup > General > Import Employee option. So you can import all your staff RDO balances at once.
You can also use this same value to adjust the RDO accumulated value should the accumulated value need adjusting.