Can a window unit air conditioner be recharged with freon? (or whatever the coolant used is called) If so, how?

Yes, you need to add a service valve to the suction line, but first you should address the problem of why it needs charged, i.e. find the leak.

NorthernHeat is correct, freon doesn't wear out or go away unless you have a leak. You need to take the cover off and make sure both coils are clean. GOOD LUCK!

I`ve found in my years that very few window ac`s ever need freon added,, a good coil cleaning usually fixes the problem, and not just cleaned from the outside, take it apart and cleaned from the inside out