Do you've toolbox installed? Is yes then check under Tools > Add-in and then create the required gear.
Before you use a tool box gear in a design, you need to do the following to get a sense of what you are about to do...
Go find something you own and value that is delicate and expensive. Hit it with the biggest hammer you can find and then throw it against a wall as hard as possible.
This is what Toolbox gears will do to your product from the inside out.
I do this often.
First, create a new assembly, and drag the gear you want into it from the Toolbox (you'll have to configure the right gear in the property manager).
Then in your assembly tree, right click the gear and open the file. Once that toolbox part is open in its own window, do a file --> save as, and save the part to your working directory. Then you can open that new copied part and modify it however you want.
Please note that this new part will still have the little "Toolbox" flag (the part icon will look like a hex bolt in your assembly tree). You can switch off this special label using a standalone exectuable file, but I can't recall the name so you'll have to search for that if it's important (likely it doesn't matter...).
Hope that helped!