In groovyConsole I have this:

import groovy.util.*
import org.codehaus.groovy.runtime.*

def gse = new GroovyScriptEngine("c:\\temp")

def script = gse.loadScriptByName("say.groovy")

this.metaClass.mixin script

say("bye")

say.groovy contains

def say(String msg) {
  println(msg)
}