This Adobe forums post appears to be the quickest route, worked great for me (don't worry about the colors changing when you get to RGB — they go back to being right on when you merge spot channels. Also note that it's really just a few steps, this makes it seem longer simply because it's an idiot-proof explanation:
The short story is: add new (blank) spot channel > move to top of stack > select it > Mode - Grayscale > Mode - RGB > select all channels > merge spot channels > save as whatever
Here's the whole spiel for your internet browsing convenience:
Originally Posted by From JWadical at Adobe Forums
Sep 30, 2011 11:50 AM
Assuming you have a multichannel file with channel structure like below:
First, create a new spot channel (using the channels panel flyout menu).
Then move it to the top of the channel stack (Ive named mine "to be gray"). Make sure the top channel is blank (all white). Also make sure that it's highighted in blue (it's the active channel).
Then choose Image > Mode > Grayscale from the menus. You should see that channel turn to "Gray".
Then choose Image > Mode > RGB from the menus. You'll see it turn from "Gray" to separate RGB channels. Now using shift, make sure all channels you want merged are visible (eye icons on) and are highlighted in blue (see below).
Then choose "Merge Spot Channels" from the channel panel flyout menu. You should be ready to roll now.