Yes you can!
First, you will need to create a "custom" device in your script's Device List. Select "Custom" as the Make and Model.
To send a simple ASCII or hex message out, you can use your show controller's "Message Out" event.
Parameter 1 of the Message Out event is the device you want to send the message to (your custom device you just created).
Parameter 2 is the message itself. You just need to make sure to put double quotes around ASCII characters. Hex characters are denoted by a lowercase h.
You can use this Message Out event to send a message to a custom device, or to an existing device if there's not an event you need built into the product file yet.