How to make a small storage in my macbook to act as a USB stick? I want to partition 128Mb out of my macbook hard drive and use it as an external usb; mainly for scanners that they can stick a usb stick into. This way I won't need to install drivers for every scanner I use.

Is that possible? Is there a small app that can make it? Convert the usb port into a usb stick on option.
You mean use the partition as a USB drive on an external scanner?  If you start the computer in target disk mode and plug it into the scanner with a USB cord, this may work.

If you're running Mountain Lion or Mavericks, you can use Disk Utility to add a small, FAT  formatted partition to your hard drive without erasing it or losing any data.

Connecting that partition to an external scanner may prove to be trickier than installing drivers for a scanner.