How to add %triggerpostun scriptlet to rpm

There are CPACK variables to add various scriptlets to an RPM (install, uninstall, trans) but no method I’ve been able to find to add a trigger scriptlet.

I have a use-case that needs to add

%triggerpostun — old-name
# old-name’s %postun ran with $1==0 and undid setup; redo it.

exit 0

How can I do this?