The entire DNN core is open source - and as such if you are willing to get your hands into the c# code there is nothing that cannot be modified.  However, that does come with a future compatibility proviso - if you change the API or edit a core module - then you will need to plan around keeping your custom modules compatible with future updates to DNN itself.

In most cases - what people do - is make a clone or copy of the existing core module - and use that as the base for your modifications while leaving the API layer alone.

Over the years we have built a number of custom sites based on this workflow - and they have evolved successfully from as far back as dnn 2.0 all the way thru to the current version - yes at times some custom modules have required a rethink or patch to ensure compatibility with new dnn core features - but the way the DNN api legacy calls system is maintained meant for the most part that things just continue to work.

Westa

Thanks matthias for providing the licensing information.