All leasing companies we have dealt with are happy for private plates to go onto their vehicles.
If you are the registered keeper, you deal with the plate transfer and just have to let them know when it is complete, so they can update their systems with the new registration number.
If the leasing company is the registered keeper, you will need to ask them to deal with it for you. They will probably charge an admin fee when they put the plate on, and another one when it needs to come off at the end of the contract.