Is there a way to create a custom keyboard in KDE?

There's unfortunately no graphical program to create keyboard layouts on Linux, except for some very old software that doesn't work properly anymore.

The only way for you to have a full custom keyboard is to manually edit configuration files. I've created a layout setup and saved it on /usr/share/X11/xkb/symbols/, and then added it to the list at /usr/share/X11/xkb/rules/evdev.xml. Logging out and in again was enough for it to be available on the KDE input preferences.

To learn how those files work, take a look at the existing keyboards on /usr/share/X11/xkb/symbols/. You can find more info online by looking up "custom xkb keyboard" on search engines. They are complicated at first, but follow a logical structure based on the location of each key (row and column).