Hello Costas,
We have a tutorial with step-by-step instructions for how to use Reaper ( http://www.reaper.fm/ ) with a VST/AU plug-in to apply real-time reverb to Hauptwerk's output here, which hopefully would get you started:
https://www.hauptwerk.com/news/tutorials-sequencing/2015/06/08/hauptwerk-with-reaper/
There are various popular third-party VST/AU convolution reverb plug-ins available.