I found a way to copy a button but it doesn't work everywhere - Select the tool and MMB drag it's icon from the tool box.

I also encountered another problem when not doing it this way: When you double-click a button it doesn't open the tool settings like in a normal button.

Is there another way to copy a button which works with every button in the shelf?
Is there a way to make a custom button open the tool settings on double-click?