That facility does not exist in the Pegasus address book (though it is in other mail clients). However, you can do it through other means. This is what I do:
If you filter messages from your mailing list into a separate folder, you can then set that folder to use a different Pegasus Identity when creating replies to those messages (right-click on the folder and choose "Set default identity for entry"). If you make sure that the identity you choose is configured to only ever send plain text messages (Tools > Options > Message Formatting > Disable all text styling options), everything will be fine.
With this method you can also exploit the other benefits of an alternative identity such as the From name being something different or perhaps sending through a different server.