89 dB is the reference number. Every single time you add ReplayGain tags it will target 89 dB.
You cannot change that reference number. This ensures that every single person on this planet will have equally loud files.
Take a look at this photo.
As you can see, it is called Preamp. This tool does exactly what it says - it acts as a preamp.
Since we know that ReplayGain targets 89 dB, and we want 93 dB, we simply have to add +4 dB to it.
If you decided to add +4 dB to files without ReplayGain info, then all of your files will be +4 dB louder
This is dumb because you have no idea how loud your files actually are.
1 dB is equal to 1 LUFS in this case.
89 dB is equal to -18 LUFS.
If you want 86 dB, then you will need -3 dB (89 - 3 = 86) or -3 LUFS (-18 - 3 = -21 LUFS)
If you want 93 dB, then you will need +4 dB (89 + 4 = 93) or +4 LUFS (-18 + 4 = -14 LUFS)
Edit: I just Googled to see how other software does it. This tool by Poikosoft looks very nice.
Maybe you can ask foobar2000 developers if they can implement similar scale to this.