The K6 is not programmable out-of-the-box, but it seems some people have successfully installed QMK firmware on it (with caveats); github.com/CanUnesi/QMK-on-K6/
If you just want to try the numpad thing out, it might be easier to use something like AutoHotKey. Here’s an example of a script that uses CapsLock to toggle the numpad layer.
Of course, AHK scripts only work with the computer you’ve installed them to, whereas using programmable firmware will work regardless of which device you connect your keyboard to.