Well I remember from the DVD's that, I think it was Jeri Taylor who mentioned it, but the producers had seen the books for Day Of Honor and liked the idea so much that they created the episode. This is one case for sure where the idea for an episode originated in the books.
That's not quite right, because the first book came out only weeks before the episode aired. Rather, when Pocket editor John Ordover was developing the books, he contacted Jeri Taylor and let her know about the idea, thinking it might be a good basis for a B'Elanna episode. And Taylor agreed.