As you might be aware, the note range OpenMPT can handle is 10 octaves (octave 0 through octave 9), just like Impulse Tracker.
That's a total of 120 notes. However, the MIDI standard has a 7-bit note range (0 through 127), which is a bit more than what OpenMPT has to offer. Sometimes, this can be a problem, for example when playing a MIDI drum kit.
Luckily, there's a way to access the full MIDI note range, although it's a bit unusual and not very straightforward.
The answer is simple: Zxx macros!
You can configure the custom macros 9c z 7F and 8c z 7F and use them on a channel that has the target VST assigned, or just play a note with an instrument assigned to this VST on the channel. The "c" in those macros sends the MIDI channel, so you can replace it by any hex number between 0 and F for a fixed channel.
The "z" defines the actual Zxx parameter (0 through 7F). "7F" is the note volume (maximum). Now, if you want to stop a note, you will have to switch to the other macro and call it with exactly the same Zxx value.
It's probably most useful to use the second macro on a different channel.
Now you can access those additional notes you probably need for drum kits and similar stuff!