Gee, great question and unfortunately I don't know the answer, my Janome has 2 vertical spool pins! Do you have those plastic round things that push onto the pin to hold the thread on, wouldn't that work? Or isn't there room on the pin for a large spool? I've seen those metal stands that people use for the really big spools.
My best suggestion is to call your closest Janome dealer or maybe ask at JoAnn's but in the sewing machine center there if they have one. That is where I buy my Superior thread, at JoAnn's in the sewing machine center, not in the normal thread area of JoAnn's. I've gotten better prices of Superior Threads when they are at the big quilt shows, they offer better deals and you can ask questions in person, see a whole wall of sample stitches of all their threads.
I think you will be so pleased with the Connecting Threads Essential, the price is wonderful and so is the thread. I got the 'bright' collection and planning on getting either the neutral or the spring set.
best of luck!
PS, I'm originally from the Canton/Akron Ohio area, I see you are in NW OH
sue