Thousands seperator conversion [IDEA]

I'd like to see an extension which like the time conversion extension automatically adds thousands seperators every three digits. For my game and I imagine many others I want to display my highscores like this:

1.000.000 or 1,000,000 if you use American style

Shouldn't be too hard but I don't know much about custom stuff so I'll leave it to someone with the necessary skills.