on a window air conditioner, can I extend it further into the room and add an extension sleeve on the back?

To some extent, but as you move the unit into the room you're also moving a "hot" item into the room, so you'll be adding a new heat source into the room, thereby making the unit work longer to remove the extra heat.

The previous answers are correct about defeating the purpose by adding hot air into the room. What they missed was that a window unit is balanced and to move it into the room will through that balance out so you will not only end up putting on an ugly extension and heating the room rather than cooling it but you will also have to build a bracket on the inside to keep it from falling. In addition you have the risk of having the humidity it removes actually drip into the room as well.

If you create a sleeve, you will need to use a heavily insulated material. Otherwise the hot air blown out the back will vent into the room and you've completely defeated the purpose.

I agree with the person above. The heat from the compressor will heat up your room and your AC will loose efficiency. The better thing to do is make some sort of ducting out of cardboard boxes and channel the cold air to where you want it to go.