Load a library dynamically rather than using @Library syntax. You may use global variables defined in the library thereafter:

library 'mylib@master'
usefulFunction()

You may also load classes defined in the library by selecting their fully-qualified names like properties on the return value of this step, then call static methods or call constructors as if they were a method named new:

def utils = library('mylib').com.mycorp.jenkins.Utils.new(this)
utils.handyStuff()