I think that the article by Dr. Kokjohn is a good followup to Ms. Rainey's devastating critique and a useful antidote to the pablum that's generally offered up as metaphysical thought.
Bill Chalker's story (told in 'The Hair Of The Alien') sounded intriguing, although the hair in question was in fact human, and the story rests largely on the perceived credibility of one experiencer. But I think that hybridizing the human race with an alien species would be difficult, even for an alien.
Whitley Strieber has suggested many times a probability that I accept, and one that's generally overlooked by mainstream UFOlogy: that a true extraterrestrial sentient species would not only be radically different from us biologically but also consciously; much of the world that we accept as "real" is largely a consensus construct, and a true alien race might perceive and construct reality in ways so different from us that it might be extremely difficult for them to interact with us on a substantial basis. Most exobiologists are focussed on possible genetic incompatibility, which might be profound, but I believe (and I think that science will eventually prove) that "reality" is, to a certain extent, a conscious construct; and the quality and nature of the reality that we perceive is determined not only by the physical limitations of the brain, but also by the limitations of the consciousness that inhabits it. Even time, which we accept as an absolute, might be experienced in radically different ways by an alien species. We simply do not know, or even know enough to speculate.
What we can assume is that the chances that an alien biological race might be able to share the same space with us would be prohibited by one simple fact: microbes and bacteria, which are abundant on our planet and bodies (and presumably on alien worlds), might be extremely dangerous to an alien body (and vice versa). Our species co-evolved many millennia with these germs and in fact could not thrive in their absence; so alien engineering would have to find a workaround for these billions of fellow travelers, or safeguards against them which would not damage the host organism.
I don't think that Jacobs and Hopkins thought these issues out in a true scientific manner, which is why their work never really passed muster with those outside UFOlogy.
This doesn't make hybridization impossible, but probably so difficult as to be useless to an alien race. There's probably many reasons that dumb animals are confined to planets, and perhaps reasons why we still continue to be.
Nonetheless, I will always argue, in the true spirit of scientific inquiry, that we can't rule anything out absolutely. To say that we do not yet know is not to say that something is impossible. I am always open to consider a good, plausible theory that can explain the vast unknown, even if the theory is ultimately proven to be crap. Science has failed in this arena by failing to entertain "alternative" theories of being, and like most rigid orthodoxies, tends to suppress views that are heretical. For me, this is the reason I am still attracted to paranormal study, despite its pitfalls. So perhaps the true metaphysician should keep a foot both in the scientific as well as the speculative world, and ever be careful not to fall onto either side.