the only thing i could think of would be scripting a reaccuring event that when you move a unit of ... hoplitai haploi with 9 Xp into a predefined city or city with a specific building, it gets replaced by a unit of Hoplitai with 3 XP. dunno if this works but I'm quite sure that it would have to be done for every unit seperately and thus beeing a HUGE pile of scripting that'd slow down the game drastically.

plus in EBII only every 3rd XP gives a bonus on stats ;).