Q: Can a memory stick be plugged into an iPad?

Possibly. You will need the Camera Connection Kit. iPad- Using the iPad Camera Connection Kit

The iPad is not really designed to do things that way, but if you insist that is what you need to do have a look at this product:

http://www.hypershop.com/iFlashDrive-by-HyperDrive-s/210.htm

Even if you get a USB stick to work - and some do with the camera connection kit - the only protocol the iPad supports natively is the import of photos and videos. ANd you can't get photos or videos off the ipad onto a stick, only onto the iPad.

For a USB to work - and only some do - you want to avoid ones like Sandisk or Kingston that have bundled software (or take that software off it). When that software runs it triggers the 'too much power' warning. You also want to avoid ones with big flashy LED lights. That too tries to draw too much power.

Any photos or videos have to follow a strickt naming convention. Be in a folder named DCIM and have a file name of only 8 characters plus extension (DSC_2344 for example)

If you want to attempt other files, you'll need one of those expensive things and read the reviews because I haven't heard much first hand experience with them.