test2.hx

class TestClass implements Dynamic { // this is the crucial part
	var a:String;
	
	public function new() {
		this.p = "test1";
	}
}

....

var testObj = new TestClass();
trace(testObj.a); // => "test"

testObj.b = "test2" // absolutely fine
testObj.c = function() {  // absolutely fine again
	return "test3";
}