How to customize a Jenkins job dsl method
What you want is a Jenkins shared library which will allow you to (among other things) define custom Pipeline DSL steps in a repository you own.  See the "Defining custom steps" section in that document.

Here's the example custom step from that document:

// vars/sayHello.groovy
def call(String name = 'human') {
    // Any valid steps can be called from this code, just like in other
    // Scripted Pipeline
    echo "Hello, ${name}."
}