Have you decided if it is really ‘epoch’ form you are after yet?
If you don’t want to create another Item to hold it as a string, I think you could write a javascript JS transform to use in the DateTime item’s label.
The javascript would get an ISO string like
2018-05-05T05:55:55.000-04:00
which you would need to parse into a javascript object, then get the milliseconds epoch to return to openHAB.
Some ideas here -