The clipboard filter is an excellent action (thank you @ComplexPoint for encouraging me to use it). The dropdown list contains a dropdown list with a myriad of possible filters as per second image below.
As far as I understand, one has to create or download from the KM Forum one macro per type of filter.
I was wondering if it
would be possible to use a single macro with a single Filter Clipboard action. Triggering the macro would display the dropdown list of possible filters which the user chooses from.
if I can add my own filters to the Filter Clipboard action somewhere in KM settings
There may well be a more efficient way to do this, but if you don't mind putting the time in... Add an entry in the Switch/Case group for each filter you'd like to include. I've filled in the first three for you.
No, you can't. But if you're using @noisneil's Switch/Case system above you can make another Case that uses either KM actions (including "Execute Shell Script") or a complete subroutine you've written, and add that to your dropdown.