It's not an addiction - I just like nice pens. Not expensive pens - nice ones. Take the MUJI pen/pencil combo. It's less than $4 and with an X-ACTO knife, a Hi-Tec-C Coleto refill and 5 minutes, I made mine into one of my favorite pens.

Step 1. Order the parts. I got the pen(cil) from MUJI's US website, and the refill from Jetpens (be sure to get the Coleto multi-pen refill). The main draw of the MUJI pen was the lack of external plungers found on other inexpensive multi-pens. I'm a Serious Professional (TM), and multi-colored plungers make me feel like I'm in grade school. However, I didn't like the ballpoint component of the pen - I mean, it's OK, but it's just a boring ballpoint. Pilot's Hi-Tec-C pens are smudge-resistant gels available in thin widths and lots of colors. Read more about them at Jetpens or The Pen Addict.

Step 2. Unscrew the barrel of the MUJI pen and carefully pull out the ballpoint pen component - it'll probably resist a bit.

Step 3. Pull the plunger out of the Hi-Tec-C refill.

Step 4. Lay the Hi-Tec-C refill and ballpoint component next to each other, tips aligned. Cut off a bit from the butt end of the Hi-Tec-C refill so it's the same length as the ballpoint.

Step 5. Reassemble the MUJI pen, being sure to press the refill in all the way. Dance around the room, because you have a sweet pen/pencil combo that cost $5.50.