I am not as knowledged as others here about our English language, but I did stumble across this:
I suppose one verb would be "singularise". It feels right to use, although I must admit I can't recall it ever being used and additionally I find the spelling to be odd.
Based of speech, I would personally spell it & sound-it-out like 'singulise' (sing-gul-lise).
Regardless though, The word sounds right to me and I would think most native english speaking people would understand the context without much problem, but either way the spelling does still looks strange to me.
the only other thing I was wondering, is whether you could simply say "depluralise" which would effectively imply the same thing.
This is a really VERY intriguing question you have raised! :)