SUMMARY

Currently, mouse navigation, if enabled, ignores NumLock, so users who frequently need both, have no choice but to toggle mouse navigation instead, imposing needless disadvantages (see below).

STEPS TO REPRODUCE
1. Turn on NumLock; verify you can enter digits by the number pad keys.
2. Turn off NumLock; verify you can move the text cursor by the number pad keys.
3. To move the mouse cursor instead, go to "System Settings" --> Accessibility --> "Mouse Navigation", enable "Use number pad to move cursor"; verify you can move the mouse cursor by the number pad keys.
5. Turn on NumLock; try to enter digits at the number pad, notice it doesn't work. 
6. To make it work, try to find an option in System Settings that allows you to use NumLock to enter digits while otherwise mouse navigation is enabled.

OBSERVED RESULT

There is no such option.

EXPECTED RESULT

There should be an option that allows you to quickly toggle mouse navigation and numerical input by the NumLock key, just like people who don't use mouse navigation can toggle arrow keys and numerical input by the same NumLock key. 
IOW, it should be possible to convert only the numpad's arrow mode to mouse navigation, while keeping the numpad's numeric input mode intact.

Preferrably, there would be a two-point radio-button just below 
> "System Settings" --> Accessibility --> "Mouse Navigation" --> "Use number pad to move cursor"
following the same structure as 
> "System Settings" --> Accessibility --> Bell --> "Visual Bell"
which is:
>        Use number pad to move cursor  [x] Enable
>                       If NumLock On:   O  Ignore (Move Cursor)
>                                        O  Enter Digits
where the radio-buttons are greyed out unless the checkbox is enabled.
(Mock-Up best viewed in monospace font.)


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: MX 23 KDE
KDE Plasma Version: 5.27.5
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION

The least cumbersome procedure to "quickly enter some numbers, then continue using mouse navigation" requires "System Settings" --> Keyboard --> Keyboard --> Advanced --> "Compatibility Options" --> "Shift + Num Lock enables Pointer Keys". You then have to press Shift+NumLock each time to toggle. 

Unfortunately, this procedure has two drawbacks:

1. Shift+NumLock is more difficult to type. Having to use this two-key shortcut makes no sense(*) if at the same time, the much more easiliy typable single NumLock key is out of function. ((*)That is, unless you deliberately want to make it harder, e. g. to prevent accidental toggling. However, if you are able to use the keyboard at all, that risk seems rather remote.)
2. The Shift+NumLock toggle provides no visual state indication, whilst at the same time, the NumLock LED status is visibly On or Off - but without meaning.

p. s. I have found https://bugs.kde.org/show_bug.cgi?id=140935 which was related, but I don't think it did a particularly good job at getting the issue across to non-affected/non-impaired developers, especially regarding the drawbacks of having to abuse Shift+NumLock as a NumLock replacement, on a daily basis. That's the main point here, and unfortunately, it still remains.