I would think your best chance of successfully transplanting a hemlock would be in winter when it's dormant. Late fall after a few frosts, at the soonest.
If it's very small, say less than a foot tall, and you can put it in a pot, keep it in the shade over the summer and water it diligently, you might get lucky with it. They grow a long, deep root as I recall so be sure you get the whole thing.
