Here's a G chord being played by the guitar, if you have a custom chord or unavailable one Biab could intelligently build on the midi notes to add more chord notes that are played by the PRT's sfz sample, Reaper can do this.
Also the piano roll should have an option to color the background to the chord notes or scale notes allowing a quick easy manual way to do this.

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BB23-Add-Chord-Notes-To-RealTracks.png (76.74 KB, 21 downloads)