Custom Keyboard Layout

[MACJAVO]

Is there a chance to edit an established keyboard layout? Say, for example, I use the Spanish layout that's a standard in every Mac. I hate though that the keypad only allows me to type commas instead of periods every time I want to type a decimal number (the . key below the 3 only types commas in the Spanish keyboard). What I used to work with was a keyboard layout that was a merge between the US keyboard and the Spanish keyboard. I used to have a custom keyboard layout but I accidentaly deleted it and now I don't know how to configure one of the preset layouts.

Is there a chance you can modify or create a new layout?

Thanks