To come from outside of "folk" and make it "into folk" the song needs to be memorable, eminently singable, held in general affection, and probably tell a bit of a story. If you're in the pub and start to sing it and some of the locals join in, it may be on its way to becoming a folk song.

The obvious one is "(Why why why) Delilah."

Ones that sometimes work:

"Johnny, remember me"
"Puff the Magic Dragon"
"Two Little Boys (had two little toys)"

One that might almost work: Lynyrd Skynyrd's "Gimme Three Steps"http://www.sing365.com/music/lyric.nsf/Gimme-Three-Steps-lyrics-Lynyrd-Skynyrd/08FCF927B631FB68482569EB0011464E

Another that nearly works: Ace of Spades (Motörhead) which fits nicely to a slight variant of the tune of the Lincolnshire Poacher.