Officially it's not possible, but it should work and I will think about another player expansion in the future :-)
Just need a new color and some tiles (one more each round). Maybe you could do this with another Peloponnes Box, just use 3 Tiles A, 3 Tiles B and 2 Tiles C.
A playtest for a new expansion will need some more time and a new idea for something special, that not only expand one more player.
PS: Athens mini expansion is now officially for sale at the shop:
For Germany: http://www.my24shop.biz/?shop=2266
For EU: http://www.my24shop.biz/?shop=2267
For outside EU: http://www.my24shop.biz/?shop=2268