Can I buy an add on educational kit for my retail EV3?
There is an expansion set, as well as a selection of additional sensors, etc. you could purchase, listed on the products page of the LEGO site's Educational Section:


The Expansion Set - Lots more elements, no electronics.
The Gyro sensor
The IR Beacon - This comes with the home set, but not the Education set.
The Core Set - no software - see "What is different between the EV3 Home and Educational sets" for more on the differences between the two.


Note that these tend to more expensive than the retail kits, and tend to have slightly different items in them.

There have previously been a number of 3rd party suppliers making sensors for the NXT and RIS systems, so I'm sure they will also provide support for EV3.