This Instructable is a variation on the one submitted by computercolin. His was a great idea that I implemented slightly differently and I wanted to share.
Step 1: Gather Materials
Staples gel ink mini pens
an empty (or as empty as possible) Cross refill
tack (for punching a small hole)
Step 2: Disassemble Mini Refill
The rest of the pen, I'm setting aside to make an iPod Touch stylus.
Step 3: Prepare Mini Refill
Step 4: End Cap
Step 5: Prepare the Widener
THIS STEP WILL BE MESSY!
However, you are only going to ever need to do this once.
It is important to find an almost empty refill for this because we are going to dump the ink out of it. You don't want to have to deal with more ink than necessary.
It CAN be done with minimal mess. I was wearing dress clothes when I did it and didn't get a drop on me (although the janitorial staff may wonder why they can't get the stain out of the sink in the men's bathroom).
1. Pop the end cap off.
2. Cut around the ridge where it narrows down, thus cutting the ballpoint tip off of the plastic well.
3. Once you've cut all the way through, put it on a paper towel and let gravity and capillary action take the ink out as much as possible. Be patient. You can try to rush it by running water into the back end, but do it slowly. You don't want to splash. DO NOT turn the pen upside-down and pour water into the narrow end (this caused considerable splashing of ink). Note even the clear gel stains.
4. When you've almost got all of it, roll up a snake of paper towel and push it through the inside to clean the last of the ink out.
5. Use rubbing alcohol to clean up messes and to get the last of the ink out. (optional)
Step 6: Widen the Opening
Step 7: Insert Pen
Slide the refill into the widener. It should be snug.
Pop the ventilated cap on the other end. You may need to grab it by the ballpoint end and wiggle it around to center it. (Here is where the Cross caps from their refills come in. I put them on so I can grab the ballpoint end without writing all over myself. Not that it matters since I'm covered in ink having just cut open a refill, but the next time, it will be much neater).
Step 8: Success!
You cannot see through the Ion window how much ink is left, but that isn't such a big deal to me since that was never an accurate indicator of how much longer my pen was going to last anyway.
Thanks again to computercolin for pointing the way and showing me how to continue to use my favorite pen while not wasting money on bad ink.