Well, the approach for training cats to do tricks, gradually shaping the behavior, might work...
She already knows the pad. Move the pad slowly to and out the door, a foot or two every few days, so she is always going go where she found the pad last time and can find it from there. If she seems to be getting the idea you might be able to move faster; if she objects because it isn't where it's supposed to be you may need to slow down or take a step back. Once you get her outside you can start reducing the size of the pad to transition her to grass.
Or go back to the puppy-training approach: feed and water, then take her outside on leash and keep her company until she has to go, then praise/reward her effusively.
You've trained her to a specific location and cue, and she's trying to obey. Now you need to retrain to tell her that outdoors is ok too... and that's going to require convincing her to break the previous training before you can confirm that, yes, this is ehat you want her to do.