How to use external reverb

as Dikrek replied you can add an FX reverb VST3 or AU  (if Mac) in a DAW or a VST Host.   
If you use this method, it is recommended to also add an external EQ as the last correction effect. This is best  mixing practice
so the chain would like  Pianoteq +EQ pre model + rendering with pianoteq model + EQ3 corrective  prior reverb effect + Magic 7 as an FX effect + final  external FX EQ to adjust final sound before output . In the above thread @Lemuel uses Fab Filter EQ  as the last effect in the chain which is quite good .

Other solution is to use the same bricasti M7 IR files (free as well in samplicity  ) instead of  magic 7 , loaded directly in pianoteq internal convolution engine and use the pianoteq EQ3 which is now a 5 band parametric EQ applied as the last effect . For live playing , I find this setup  very responsive  compared to the above solution . Ideally make sure you choose bricasti wav files with the sample rate being equal to the sample rate of your audio set up  . Bricasti files are available in 44.1 kHz and 48 kHz . This avoids unneccessary conversion of sample rates  when different and saves CPU time.