Or, you can take a a short piece of 1/2" tubing and flare one end. Ream out the other end of the 1/2" tubing, and slide it into a piece of 3/8" tubing which has been reamed out. Braze the slip joint together and you're done.

When you are drilling out the hole for the 1/4" stub as Peter suggested, take a center punch and make a slight impression on the tubing where you want the hole to be. Otherwise the drill bit will skip all over the tubing.