@merox If you want to experiment with installing new packages, I think the best bet is to build your own package. The source code for the nextcloud package is at http://git.cloudron.io/cloudron/nextcloud-app . You can build that package by following the tutorial at https://cloudron.io/documentation/custom-apps/tutorial/
What you would do is to add the additional package you want at https://git.cloudron.io/cloudron/nextcloud-app/blob/master/Dockerfile#L6 .
Would that work?