I don't think it's outside of the realm of possibility, but I don't think the tech is quite where it needs to be yet. I'm an electrical engineering technology student at Purdue, and we do a lot of research and work with EVs. In fact, many of my classes are based around them in some way, since it's such a huge thing in our field right now.
So far, there isn't much in the way of retrofit kits, probably because of high cost and low demand. It also doesn't make much sense to simply replace a gas / diesel engine with an electric one and use the rest of the drive train. Electric motors don't really need a transmission, and it's much better to have them power a single wheel or axle directly.
Now, a bus is a pretty good platform for a conversion I would say. Lots of room for batteries and other components, lots of weight capacity, etc.