From The Kingdom of Loathing:
The Pretty Good Escape Subject 37, in the Cobb's Knob Menagerie, wants some spit from one of the weremooses on Level 2. He also wants to know the right way to pluralize "weremoose."
Since it is a word created simply for the purposes of that particular game, only the creator would know. However, the plural of the word moose is moose, as attested by Merriam-Webster.
I would suggest weremoose would be the most likely answer to your question.