Possibility of adding a numeric keypad?

[nullvalue0]

Looking at this page, it appears the numeric keypad resides at matrix ROW 9 & 10 (UK matrix format). I see in the keyboard schematic, on the 74LS145, Q9 is not hooked up which might get me one extra row, or half of the keypad keys. What might it take to fully support the keypad? (I'm thinking about designing an add-on PCB)

[skiselev]

It would require changing the decoders. Perhaps similar to this: https://github.com/vipoo/yellow-msx-series-for-rc2014/blob/main/keyboard/schematic.png
The main reason I didn't implement numeric keypad is to keep the keyboard PCB size small. But, as you've mentioned, it is possible to add a connector for an optional numeric keypad board.
I'll keep this in mind for future redesigns.
Or, if you feel so inclined and you have a bit of time, perhaps you can take the schematic linked above, and modify the keyboard PCB layout accordingly, adding a connector with rows 9 and 10, and all columns 0 - 7 (so, 10 signals, perhaps using a 5x2 header?