Actually there was a Human/Eldar Hybrid named in a Rogue Trader Book. And he was an ultrasmurf
But then again canon is all sorts of fucked (*hears slaneesh hissing in pleasure*) So I guess its more up to what you want at the moment/for the setting
That is no longer canon alas, like much of Rogue Trader it was dropped in later editions as they figured out the setting. At that point it wasnt 40k so much as WHF IN SPACE!
The current canon is that not only do human and eldar have wildly different genetics (you have more in common with a jellyfish than a eldar), but also things like the specifics of conception (humans fertilize and done, Eldar make several distinct contributions to produce a viable fetus) and the fundamental materials that make up the body (eldar teeth are not enamel, their bones are not calcium, etc) mean that a human would have have about as much luck trying to crossbreed with a quartz deposit, even in a lab experiment.
Now... if one wanted, one could certainly ignore canon and do what they want... 4chan certainly does, but the canon is not ambiguous on the subject.