I recommend going a separate numpad: https://www.reddit.com/r/MechanicalKeyboards/wiki/numpads

Once you're willing to separate, the options WIDELY open up.

There is not such a kit. In this day and age of hand-wired keyboards and 3D printers, though, you can probably make it.

I use an Ergodox with one one of the bottom keys set to switch modes while pressed; in that mode m,.jkluio translates to 123456789. It works fine for me.

The Ergodox has too many keys, so using some of them to switch layers is practical.

No. But a keypad PCB is 5-10 USD and well worth it.

I need it for my daily work and I can't really map it well on my 40%.

Because there is regularly no need for these. You can add num block to any part of the keyboard and switch to it with one key press(Hold or Tap) You can also get separate and programmable num block in addition if you really want it.

On my ergo I use the home row for the numbers. It's way faster because you can use 8 fingers for 10 numbers instead of 3 fingers for 10.

Of course there is no need for them but I am just used to use one and I would miss it.

I wonder if at that point users aren’t just emulating a ten key with some mode switch, or just building a separate ten key?