So, if you are lucky enough to find a White Pilot Choose pen on Amazon (they go out of stock quick) here's a way to adapt Uni-Ball Signo pens using the Pilot Choose case.
Because the Pilot pen is thinner than the cricut pens, I posted earlier to use pieces of paper around the pen to center it in the pen adapter of your machine.
I've found that taping around the case works much better, gives it a snug fit and lasts a lot longer (I prefer masking tape because it's thicker and somehow resilient).
The Signo pens are larger, so it is better to take the refill and cut it so it will fit the Pilot case without popping the end cap every time you insert them. I like to use an exacto knife, as scissors may flatten the end of the refill.
Just make sure you are cutting it above the clear gel at the end, which prevents the ink from bleeding and drying.
Take the Signo refill and put it in the Pilot case, insert it in your machine's pen adapter and start writing!
In case you run out of white ink in your Pilot pen, just buy a Uni-Ball Signo White Gel Pen - a lot easier to find in stock. They came in Metallic (the ones featured here), Angelic (pastel colors, this one includes white), and Glitter. All links to Amazon are at the end of this post.
Here's some tests made with the metallics, Enjoy!