You will need:
1 (starving) Cross Ion Gel pen
1 Staples Mini Gel Stick pen
1 Alligator style staple remover
1 File (pick your favorite tooth configuration)
Step 1: The Problem
Step 2: Prepare the Stopper
Luckily, the Staples Mini Gel Stick pens have an end cap that is very similar to the one found on the Cross Ion refill (See the plastic end piece I removed from the Cross Ion refill).
*Grab your alligator style paper clip remover and pull out the stopper from the body of the Mini Gel Stick pen.
But don't go shoving this on the end of the gel stick just yet; without an air-hole, you'll just increase the pressure in the stick until it releases in a black puddle of ooze, all over you hands (trust me on this one). On the other hand, if you're gel pens are dead, this might be a good way to revive them.
*Using the tack, poke a hole through the end of the stopper.
Step 3: Obtaining the Correct Length
*Lay the file on the table, and grind down the end of the stick. We need to get rid of about 3 mm in length.
I have no idea what kind of file this is, something for woodworking (I think). Something course is preferable, but if you're strapped for tools, I imagine that sandpaper would even do the trick.
Step 4: Fix the Stick
This cause is the shape of the Cross Ion spring. Because the Cross AT refills are fat and taper down to a ballpoint tip, the spring has the same, almost cone-like shape. The Staples refills are straight sided and don't play nicely with the original spring.
James (jameslisi) has a solution. He suggests using a second Staples end-cap to increase the width of the Staples refill.
Paraphrasing James: Take an extra end cap from the Staples pen and cut off the rounded top so that it forms an open cylinder. Put this cylinder over the Staples gel stick and bring it all the way down to the flange near the tip of the pen. You might need to use a little force when you get to the flange. So far the Staples refill with this modification has not hung up on the Ion spring.
So add this cylinder "widener," and you should be in business. Thanks James!
Step 5: Assemble
-Hopefully you followed "Fix the Stick" and broadened the Staples refill (pic soon)
*Load and lock your Cross Ion pen
Step 6: Test
Oh, and as an added benefit, you now have a greater selection of (admittedly elementary school-girly) colors.