Hi, what you did sounds correct for the firmware part. But you will also need to modify the UI if you want those pins to show up in the UI. In the UI code, the free pins are hard-coded, and not transferred from the firmware to the UI, hence you have to modify the UI code:
https://github.com/OpenSprinkler/OpenSprinkler-App/blob/master/www/js/main.js#L5038
You can download the UI code, unzip it, and run it locally in a browser.