Hi Wolflow,
Currently you’ll need to add that marketplace pack to an existing UE5 project and then Migrate(Right click on folder/asset in Content Browser → Asset Actions → Migrate) from the UE5 project into your UEFN project.
The ‘Content’ Folder in your UEFN project is at ...\YourProjectName\Plugins\YourProjectName\Content
Eventually we’ll be able to add packs directly to UEFN with ‘Fab’.