In principle it is possible using the S99 command but this absolutely not recommended because the reverb settings are not stored in the file. Someone else listening to the file (or even yourself, using a different OpenMPT installation on the same or other computer) could be hearing a completely different reverb than intended. If you do not like to use VST plugins, you should use the "WavesReverb" or "I3DL2Reverb" in OpenMPT, because those ship with the application and will sound the same on every computer. This is conceptually comparable to Chibi's reverb.
