[Aetae]

Мб так:

function MyArray () {}
MyArray.prototype = Array.prototype;
MyArray.prototype.myFunction = function () {
	return this[0] + this[1]
}
 
var myArray = new MyArray()
alert([ 
	typeof myArray.myFunction,
	myArray instanceof Array,
	typeof myArray
])

myArray.push(5)
myArray.push(3)
alert([
	myArray.length,
	myArray.myFunction()
])

...но зачем?