Yes. While naturally, each piece of gear can only have one resistance stat, you can reforge to make it have as many as you want.

So if a player reforges and have +2% to each of the 5 on all pieces, he'd gain +20% to all elements. And if he is earth main which is weak to lightning, this is what you'd get.

This does mean that they only have one slot left for other stats on each piece of gear thou...