Is there any possible way that hybrids can mate?
This is a hypothetical scenario which I think could be compared in parallel to hybrids like a mule (offspring of a male donkey and a female horse) or a hinny (offspring of a male horse and a female donkey) or zebroids (reference). In the case of the fertility of mules on wikipedia it is stated that 


  A few female mules have produced offspring when mated with a purebred
  horse or donkey.


with examples being given as 


  Since 1527 there have been more than 60 documented cases of foals born
  to female mules around the world. There are reports that a mule in
  China produced a foal in 1984. In Morocco, in early 2002, a mare mule
  produced a rare foal. In 2007 a mule named Kate gave birth to a mule
  son in Colorado. Blood and hair samples were tested verifying that the
  mother was a mule and the colt was indeed her offspring (reference).


In the case of a hinny fertility is much more rare but there is a documented case on Wikipedia


  In China in 1981, a hinny mare proved fertile with a donkey stallion.
  When the Chinese hinny was bred to a donkey jack, she produced "Dragon
  Foal," who resembled a donkey with mule-like features (reference).


The wikipedia page on hybrids will also be of interest in your research. In short, hybrids can procreate and give birth in rare cases. However, I wouldn't vouch on the survivability of the offsprings. Since your work is fictional, it is entirely left to your discretion on that topic. Btw, humazee or humzee makes a more intriguing name maybe. Good luck with the book.