It is absolutely possible to open the eMac and replace the DVD-ROM/cd-rw with a superdrive.
DVR-106, 107, and 108 will all work (although the 108 and newest may take a small driver to get OS X to like it with finder burning and such- macsales.com has this if needed.)
You realize you void warranty, must practice precautions to avoid electric shock, I'm not responsible if you zap yourself and cause serious injury, etc.
But it's possible.
For a while, there was a company whose business was to take eMacs and upgrade them for you. Apple put a stop to it when they threatened to yank said company's license to sell new macs.
You probably should consider going with the dvr106 if that's what shipped in eMacs of your model that did come with superdrive- I can't recall off the top of my head at 7am if the 107 and 108 drives are longer and would or wouldn't fit, although my inclination is to believe that they would.