[Gurthang]

I doubt they could reproduce. They were made for one purpose, to destroy other beings. They had no thoughts of love or pleasure, just kill, kill, kill.

And if they could procreate, what would have stopped them from multiplying enough to overrun the world by themselves. They were extremely powerful(When banded together, they defeated Ungoliant, who had it in her power to contend with a Vala) and if they could spawn, then the ensuing force could have easily overwhelm any others.

The simple answer is no, they could and did not.