That’s a simple question with a lot of baggage.
Open the game in dev mode. Open the events editor. Make your event. To get it to show up you have make a button in another event that links to your event. Then you should preferably have your event link back to another event so you can easily get out of it.
To add an item you’d need to use the items editor. Save it. Then you need to add it to the game either through a line of code that gives the item, or add it into a shop.
How to make make breasts expand without lactation? That’s going to require you to look at the organ code with the organ editor and try and make sense of it. Generally, you’ll need to add fat, milk, or implant volume to the breasts to make them bigger.
I encourage you to poke around the game as an example of how things are done. That better learning/teaching than i can provide.