I don't think there's a way. sorry.
from the fed ex website:
Can I leave instructions to have a FedEx Express package left without a signature, if I am the recipient?
For your security, FedEx couriers will not leave packages without a signature unless you or the shipper have an agreement on file with FedEx. If you wish to establish a permanent signature release on file, you may submit a Signature Release Form, available at any FedEx shipping center near you.
If you miss a normal package delivery, FedEx couriers will automatically reattempt delivery three times before returning your package to the shipper. On each delivery attempt, if you are not available to sign for the package, the courier will leave a Delivery Notice, usually on your front door. You have several options at this time, as follows:
Sign the Delivery Notice on the space provided and leave it where the FedEx courier can find it. The courier will then leave your package on the next delivery attempt.
Complete the signature release form on the back of the Delivery Notice, and either leave it where the courier can find it, or place it in a nearby FedEx Drop Box. You will then be signed up for permanent signature release.
Call 1.800.GoFedEx 1.800.463.3339 to arrange to pick up your package at a convenient FedEx World Service Center near you.
Exceptions:
Some authorized shippers may ship packages with the "Signature Release" section of the airbill or shipping label checked off. In this case, packages will be delivered without the need for your signature whether or not you are available to sign for the package.
Some shippers will not allow FedEx couriers to leave packages without a signature regardless of permanent signature release status.
Packages with a declared value over $500 US$ are ineligible for signature release. In these cases, FedEx must receive a valid signature. This is also true for several service options, like COD (Collect On Delivery), Dangerous Goods, etc.