They should be able to do it with not a problem at all (trust me, I work for FedEx). Call the CS # ( 1-800-463-3339), give them the tracking information and specifically ask for the agent to change the package from Delivery to Hold. A customer service queue will automatically generate at your local FedEx station, where the driver should be informed that the package is to be held at the station before it goes out on road on the delivery date.
This is not guaranteed that it will happen, though, but 99% of the time we drivers are more than grateful to have a package held at the station (less deliveries is less work
Also, FYI, even though a package has a release signature on it,
most drivers wont leave the package outside the door if it is perceived to be an expensive item and/or in danger of being stolen.