With a POD that early, of course it's possible. But though the POD may be that early, the creation of the empire will probably take place later. You can set events in motion that lead to an earier degradation of the old system of competing (leages of) city-states, or you can have some ATL war with the Persians cause the Greeks to unite under a single power which then stays in place and eventually grows into a true empire.
This is all possible. The chances are, however, that you'll see an empire later on. Why? Because the prevailing political conditions of the period of competing (leages of) city-states made it inherently difficult for any one of them to attain true hegemony over all the others. But in the period that these old structures were in decline, avenues were opened up for ambitious conquerors and unifiers. We see this in OTL. Philippos II subdued the decaying order of Greece with a finality the city-state Greeks had not been able to imagine beforehand. And his famous son Alexander was a true imperialist. Theirs was the era wherein the foundation of a true Hellenic Empire really became a likely outcome.
Some possibilities include Jason of Pherae succeeding in his ambitions to create a hegemony over all the Greeks, with Thessaly taking the place that Macedon later would in OTL (under Philippos II). Jason, or one of his heirs, could then take on a title that woyuld be equivalent to 'Emperor'. Similarly, we could see Philippos II himself living longer, winning a war against Persia, but accepting a peace offer where he gets Anatolia, the Levant and Egypt (the same offer that Alexander rejected in OTL). This would settle the issue, and assuming Philippos can make his real stable, this would give him and his heirs control over a "greater Hellenic sphere", without all of Persia involved. The whole area could be gradually Hellenised and integrated, slowly coalescing into a true Hellenic Empire. There is also the option of a slightly more cautious/reasonable ATL version of Alexander himself, who accepts the peace offer and then does the exact same thing as I just described his dad doing.
All these options could lead to a Hellenic Empire. It's not strange that they are clustered so closely together. The time was right for such developments. In OTL, it of course led to the Hellenic Period with its warring kingdoms. But there lies another option that I see leading to a Hellenic Empire: the Greek kingdoms of the era were going through a war of attrition, not unlike the Warring States Period of Chinese history. In OTL, Rome swooped in and established hegemony. In an ATL where Rome gets screwed early on, I fully expect the "Hellenic Warring States Period" to end with one of them winning out over the others, and establishing an Empire.
So, the short anwer: it is possible with a POD in 500 BC or so, but it is way, way more likely with a later POD.