I wouldn't be using the norton lapping plate, It will leave marks on the Jnat.. Not to mention the norton lapping plates do need lapping themselves.
The DMT lapping plate works well if the stone is very dished although the scratching it leaves behind is not suitable for razor friendly Jnats. You would need a worn 325 to remove the scratches after.
A synthetic nagura on a natural stone? Doesn't make much sense to me. The Jnats break down in to extremely fine particles while honing, synthetic stones don't do this as well.
Genuine nagura's aren't expensive but if you don't want to buy one you can use the DMT to make a slurry. A medium and fine nagura would be the better option though.