can someone explain the purpose of the expansion tank and where the hoses go to. My car did not have one when I bought it and I would like to put one on.

It's just there to catch and hold any excess gasoline or vapors that are present at refueling. There's one or two 4mm hoses that connect it to the overflow. The parts book isn't too clear on the routing, but it looks like it may connect to the filler neck. A larger hose connects to the charcoal canister.

Yep, the smaller hose (s/b black)runs from the filler neck to the expansion tank, on the passenger side. Drivers' side hose (larger, s/b green) runs fro the expansion tank to the charcoal canister. Not merely for when refueling, though - works continually.
By the way, did I recently read that replacement canisters are NLA?