It is possible to use/create extensions for QlikView 12, but there is not so much community fokus on QV extensions compared to Qlik Sense extensions.
However there are still some old open source extensions available on internet, not many that are maintained. There is one unmaintaines archived Sankey project by @Brian_Munz that might still be working on QV12. You can check it out at: https://github.com/brianwmunz/d3sankey-QV11
