[Plugin: SimpleModal Login] How can i change the shortcut CTRL ALT L to another one?

You’d have to open the theme js file you are using (default.js, osx.js, or your custom one) and change the following:

keydown: function (e) {
	if (e.altKey && e.ctrlKey && e.keyCode === 76) {
		$('.simplemodal-login').trigger('click.simplemodal-login');
	}
},

You didn’t say what you would want the shortcut to be, so you’d have to determine what would go in the if() condition.