Is there any camera attachment where I can change my aperture?

I have a Nikon D3100 and i don't want to buy a new lens just to get a smaller hole aperture. Is there any attachment i can but for my camera or for my lens that will allow me to do it?

Smaller hole? Do you mean aperture?

The lens you got with your D3100 is the 18-55 mm and it already has a minimum aperture that is pretty small, f/22. There are very few lenses with smaller ones.

Do you understand how the f/stop is calculated?

If you have a 50 mm lens with an aperture of 25 mm, you have lens with a widest aperture of f/2 50 / 25 = 2

There would be NO kind of attachment to make the largest physical aperture larger, how would that supposedly happen?

Wow. Just wow

You'll be happy to know you can set your lens to f32, if you have the kit lens. That's about as small an aperture as you can get these days.

OH deary me.

Firstly, Read your manual.
Secondly, buy a book on how cameras work
Thirdly, buy a book on basic photography.

I don't think you know anything about photography. You're looking for an attachment to make your aperture smaller. Why? Does f/22 not give you enough depth? Or do you want it to be darker. In that case get a neutral density filter.

Aperture means hole. Most modern lenses include the iris mechanism that controls aperture size. You can't buy it separately.

Most lenses top out at f/22. Some go to as far as f/32. Either way, both sizes are already too small to use in most ordinary shooting conditions.

If you are thinking about smaller f/numbers, those actually are larger aperture sizes. In that case, there's no other way but to buy a lens with a larger maximum aperture size (smaller f/number).