No, unfortunately. Although the individual cities existed and are likely to have been grouped together in some fashion around the time of Alexander, the New Pauly reports:

Archaeological investigations, however, have shown that the development of many towns into urban centres began only under Seleucid and Ptolemaic administration. After encroachments and occasional rule by Hasmoneans and Nabataeans in the 2nd and 1st cents. BC, the Dekapolis probably did not come into being until after Syria had come under Roman control in 64/63 BC (adoption of a new era on coins of the Dekapolis). There are, however, no indications pointing to the foundation of the Dekapolis as a political, military or economic league of cities. The member cities also had no unified administration in common.

The earliest references to the Dekapolis by name are in literary sources, not epigraphic or numismatic: the earliest definite references are in Pliny, Natural history 5.74, and the New Testament gospel of Mark, 5.20 and 7.31. We don't know which is earlier: Mark is generally dated around 70 CE, some scholars putting it a little earlier, others a little later, depending on assumptions; Pliny died in 79 CE, but we don't know exact dates for his writing.

However, Pliny does imply the existence of the name in earlier literary sources: he writes

... the Decapolitan region, named for the number of towns, though not all include the same ones in it.

'Not all' clearly indicates that he's got multiple lists of towns. In book 1, where he names his sources for each book of the Natural history, he gives 15 Roman sources (of whom the latest may be the emperor Claudius), and 45 non-Roman sources ranging from Hekataios (6th cent. BCE) to king Juba II (early 1st cent. CE): there isn't enough information to determine which sources, exactly, he was looking at for his lists of towns in the Dekapolis.

So we can reasonably infer that the name is older than Pliny, but we don't know how much older.