You can import the xtrafunction library by clicking on the menu File > Extension Management > Extensions. There you can import them. For more information about importing extensions you can refer to the section [Importing Extensions](https://community.denodo.com/docs/html/browse/latest/en/vdp/administration/importing_extensions/importing_extensions) of the Virtual DataPort Administration Guide
As for GeoJSON I am assuming you want to use the Denodo Geo service to query Denodo views and return GeoJSON objects, which is a web service you have to connect to via your browser. Before doing that you have to run the necessary start up script denodo_geo_service[.bat | .sh] in the bin folder to launch the application.
I suggest taking a look at the [Geo Service User Manual](https://community.denodo.com/docs/html/document/denodoconnects/8.0/en/Denodo%20Geo%20Service%20-%20User%20Manual ) for a more detailed explanation and also because there are alot of default properties that you can adjust.
Hope this helps!