dc3's Bearing Wheel Mods!
dc3 has contributed 3 different, all excellent mods... all with pictures! He is the man! Shake his hand warmly next time you see him. His mods can be found here:

Bearing Wheel using Team Losi parts

Bearing Wheel using Renegade parts

Bearing Wheel using Duncan parts

You need Machine screws that are about pro yo axle size and nuts (I got a set of 8 for 89 cents), a Duncan Wheels, Fire, metal skewer, rubber handle pliars, Yomega bearings and spacers, and super glue.

• fully disassemble the yo-yo, take the axle and throw it away.
  
• super glue the axle into the cap, then snap the black shell onto the cap, straighen the axle and allow the glue to dry for 20 mins.
  
• put the nut in the pliars and burn the nut for 35 secs, then push the nut onto the cap until it is flush with the edge of the axle slot, allow it to cool, then snap on to the black shell.
  
• burn the spacer for 35 secs and push it into the black shell until it is flush with the shell.*
• do the same with the other half of the yoyo.
• snap the shell on the cap, you should have 2 yo-yo halves now.
• heat up the skewer and stick it in the non-axle side of the yo-yo ( through the spacer hole) so that the axle can get into the other side of the yo-yo.
• put in bearing, put in a slick 6, screw together and yo.
  

Find someone that works in a machine shop to do it for you... having made four myself this is the only way to go...

1: Don't throw anything away (you can use the metal axle). You'll need it later.
2: Split the haves and have each one end milled for the spacers (this is much more precise) or you can make a custom jig for a drill press (use an end mill not a drill bit)
3: Now take the metal axle cut it in half and turn it down so each one fits flush with the bottom of the holes you just milled out (a metal lathe is ideal here).
4: Now drill and tap the metal axle pieces (m4 X 0.7 metric tap) This is the same size as a Proyo axle.
5: Find a Proyo axle or a 4mm bolt and cut the head off. Make sure the end does not have burrs or it wont go into the axle pieces you just tapped.
6: Install 4mm bolt, 2 brass spacers and a bearing (Proyo style).
7: Now you have a Super SMOOTH, Precise DUNCAN bearing Wheel....

Ingredients:
1 Duncan Wheel
1 Drill press with 3/8 in. spade bit
1 Team Losi bearing
2 Team Losi spacers
1 Proyo axle
1 Proyo nut
1 Bottle of clear nail polish
1 Dremel moto tool

Here how you do it:

1) Bust your wheel apart, discard the axle, separate the halves and caps.
2) Apply 3 layers of clear nail polish to the starburst of the yo-yo.
3) Use geometry to find a way to drill into the exact center of the Wheel (or construct a jig).
4) Using a 3/8 inch drill bit and a drill press, make a hole in each half of the yo-yo just deep enough for the spacer to lie flush with the surface.
5) Using a hammer, lightly tap the spacer in place (will be a tight fit).
6) Stick a Proyo axle through one half.
7) Put the bearing on the axle, and then the other half.
8) Screw a Proyo nut onto the axle and tighten the yo-yo.
9) Play with the yo-yo to see if it needs adjustment (you can open up the gap with an o-ring under the spacers).
10) If the caps do not fit on either half use your dremel to make them fit (take about an eighth of an inch off of the center).
11) If desired add weight rings from Tornado 2's (should fit on inner rim).
12) Use super glue or epoxy to secure the caps (the nut should be trapped under the cap).
13) Lubricate as desired.