I can't offer transition days/hours unless I have an empty space. If a client wants to use that empty space to transition, then their start date needs to be agreed to be earlier than their needed date and they are welcome to use the space they are paying for and attend a full or part day.
There's no option for an hourly rate for an empty space nor a reduced rate for an empty space. If they are using it, they are paying full rate for it.