this plugin provides an example of how to turn a groovy function into a global variable so that it can used from inside scripted or declarative pipeline.
However it does not implement the Step API so does not appear on the Pipeline Syntax / Reference pages or the Blue Ocean editor.
It would be nice if we could get the blue ocean REST API implementation to discover these 'groovy steps' (or pseudo Steps) using some mechanism.
e.g. maybe we could detect these GlobalVariable extensions https://github.com/jenkinsci/simple-build-for-pipeline-plugin/blob/master/src/main/java/org/jenkinsci/plugins/simplebuild/SimpleBuildDSL.java#L33
and add them as steps to the 'add step' wizard.
We may need some other interface / method / annotation so that a GlobalVariable extension could also expose the java class to be used for the DescribableModel to generate the name / display name / help of the step plus the list of parameters, display name, help, type, default value, CSS styles for editing etc.