Depending on what you want to do, there may be shortcuts already available under Shortcuts > KWin in System Settings, though you may have already checked for those.
What you could potentially do, is create a KWin script (or multiple scripts if appropriate) and as part of the script(s), register shortcuts that would show up under Shortcuts > KWin. An example of a script that registers a shortcut is in another post I made for a script to clear the “Demands Attention” status of all windows.
There is documentation for the KWin Scripting API, though keep in mind that it’s for Plasma 5 and I believe the API will have changes with Plasma 6, however I’m not aware of there being documentation available for that yet.