Growing a ladder from an object?

[reina]

Yes, use Mixen’s answer if it’s a Tiled Sprite.

If your ladder is a Sprite type you might wanna look into Image Mask extension:

[Solved] Is there such a thing as image masks (not for collision)

There is a mask extension in the extensions list. By using a red sprite you can mask other parts of another sprite.

Or, just draw the animation for the ladder appearing from up to down.