How to make a small storage in my macbook to act as a USB stick?
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.