Why would you want to? If you're worried about the integrity of the downloads, you should know that MacPorts will also download and verify a detached RSA signature for each binary archive it downloads – the public key used for verification comes to your local machine using a tamper-resistant path (either an installer .pkg signed with a DeveloperID certificate, or – in case of key updates – signed with the old key).
For distfiles (i.e. source tarballs of software MacPorts wants to build), multiple checksums stored in the Portfile need to match for MacPorts to accept the download as authentic. The Portfiles also come to your machine using a tamper-resistant path.
Replying to your original question: You don't, the mirror infrastructure doesn't support it.