Are you trying to build a "touring bike" in the technical sense, or do you just want a bike to tour on? If it's the latter, just get on your bike and ride. I have used the same GT Outpost not only for mountain biking, but for commuting, recreation, and touring for 20 years now. All I did was add a rear rack, panniers, and some slick tires.

Don't get caught up in the hype and the lingo. As ironwood said, you can tour on just about any bike.