In order to add a custom plugin to DataFusion (considering that you have already implemented it), you have to follow the steps bellow:
1) Click on the + button
2) Click on upload in the Plugin part
3) Drag your plugin JAR to the box, click next and then drag your plugin JSON to the box. You can alto just click in the box and select your JAR/JSON. After that, just click in finish.
I hope it helps