I asked a similar question awhile back, as I use Pianoteq in standalone mode. I received some very useful replies. I wound up downloading the Samplicity Bricasti M7 impulse reverb files - for free! They can then be loaded into PTQ. In the reverb section of the effects menu, the list of available reverbs includes an option to "use WAV impulse", which you can then browse to the impulse reverb file of your choice.

Here's where you can download the files I got:
https://samplicity.com/downloads

And here's the forum thread:
https://forum.modartt.com/viewtopic.php?id=12881

In my opinion these Britcasti M7 IRs are probably the best Ive heard out there for free IR files, thanks for the recommendation.

Here's where you can download the files I got:
https://samplicity.com/downloads

And here's the forum thread:
https://forum.modartt.com/viewtopic.php?id=12881

So we are saying you use Pianoteq as an instrument in a DAW, then with the FX described. I have Cubase, so I can try that.But... I use Pianoteq in Dorico and I do use some EQ and reverb there. I had the incorrect impression that people were implying you could somehow plug an external unit into Pianoteq itself and it could use that.

No it can't

In this post @Lemuel recommends Magic7 reverb:

https://forum.modartt.com/viewtopic.php?id=12870

How do you use an external reverb with Pianoteq?

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.

In this post @Lemuel recommends Magic7 reverb:

https://forum.modartt.com/viewtopic.php?id=12870

How do you use an external reverb with Pianoteq?

A DAW is the easiest. There are free options

https://forum.modartt.com/viewtopic.php?id=12870

How do you use an external reverb with Pianoteq?