People seem to be missing the forest for the trees here. While you might be able to put a 29er fork on a 520, the frame isn't built to take the stresses that a suspension fork puts on it. About the shortest travel 29er fork you can find is a 100mm travel fork. That puts a lot of stress on the head tube/top tube/down tube junction. Mountain bikes have been designed to take the stress without breaking apart. Road bikes...even steel ones...aren't built to take those kinds of stresses.

Another thing to think about, you don't want to buy a cheap 29er fork...or cheap suspension fork of any kind...you want something that at least has a lock out so that you don't have to deal with the bike inchworming down the road when you don't need it. You also want a durable fork. None of that is cheap.

It would probably be cheaper to just add drop bars to your mountain bike and ride it. It would certainly be more up to the job if you feel you need suspension.