How can I repair damaged epoxy grout in a shower?
It is usually always better to use like materials when fixing something.  If you use regular concrete based grout to patch a hole in epoxy grout, not only can you not be sure of a texture and color match, there is the risk that different expansion rates will simply cause the concrete to crack.

Not to mention, concrete grout is not stain proof, nor water proof.  Epoxy grout is both.  So eventually the difference will show...